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Question 12 Marks
Which of the following are perfect cube?
456533
Answer
On factorising 456533 into prime factors, we get
456533 = 7 × 7 × 7 × 11 × 11 × 11
Group the factors in triples of equal factors as:
456533 = {7 × 7 × 7} × {11 × 11 × 11}
It is evident that the prime factors of 456533 can be grouped into triples of equal factors and no factor is left over.
Therefore, 456533 is a perfect cube.
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Question 22 Marks
Find the cube root of each of the following natural numbers:
74088000
Answer
By prime factorization method,
$=3_\sqrt{2\times2\times2\times2\times2\times2\times3\times3\times3\times7\times7\times7}$
$=3_\sqrt{2^3\times2^3\times3^3\times5^3\times7^3}$
= 2 × 2 × 3 × 5 × 7 = 420
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Question 32 Marks
What is the smallest number by which the following number must be multiplied, so that the product are perfects cube?
7803
Answer
On factorising 7803 into prime factors, we get:
7803 = 3 × 3 × 3 × 17 × 17
On grouping the factors in triples of equal factors, we get:
7803 = {3 × 3 × 3} × 17 × 17
It is evident that the prime factors of 7803 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 7803 is a not perfect cube. However, if the number is multiplied by 17, the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 7803 should be multiplied by 17 to make it a perfect cube.
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Question 42 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?
1600
Answer
On factorising 1600 into prime factors, we get:
1600 = 2 × 2 × 2 × 2 × 2 × 2 × 5 × 5
On grouping the factors in triples of equal factors, we get:
1600 = {2 × 2 × 2} × {2 × 2 × 2} × 5 × 5
It is evident that the prime factors of 1600 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 1600 is a not perfect cube. However, if the number is divided by (5 × 5 = 255 × 5 = 25), the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 1600 should be divided by 25 to make it a perfect cube.
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Question 52 Marks
Find the cube roots of the following integers:
-5832
Answer
We have,
$=\sqrt[3]{-5832}$
$=-\sqrt[3]{5832}$
To find the cube root of 5832, we use the method of unit digits.
Let us consider the number 5832.
The unit digit is 2; therefore the unit digit in the cube root of 5832 will be 8.
After striking out the units, tens and hundreds digits of the given number, we are left with 5.
Now, 1 is the largest number whose cube is less than or
$\therefore\sqrt[3]{5832}=18$
$=\sqrt[3]{-5832}$
$= -\sqrt[3]{5832}$
$=-18$
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Question 62 Marks
Find the cube root of each of the following natural numbers:
343
Answer
By prime factorization method,
$3_\sqrt{343}$
$=3_\sqrt{7\times7\times7}$
$=7$
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Question 72 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?
8640
Answer
On factorising 8640 into prime factors, we get:
8640 = 2 × 2 × 2 × 2 × 2 × 2 × 3 × 3 × 3 × 5
On grouping the factors in triples of equal factors, we get:
8640 = {2 × 2 × 2} × {2 × 2 × 2} × {3 × 3 × 3} × 5
It is evident that the prime factors of 8640 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 8640 is a not perfect cube. However, if the number is divided by 5, the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 8640 should be divided by 5 to make it a perfect cube.
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Question 82 Marks
Show that:
$\sqrt[3]{-125-1000}=\sqrt[3]{-125}\times\sqrt[3]{-1000}$
Answer
$\text{L.H.S}=\sqrt[3]{-125\times-1000}$
$=\sqrt[3]{-5\times-5\times-5\times-10\times-10\times-10}$
$=\sqrt[3]{\{-5\times-5\times-5\times\}\times\{-10\times-10\times-10\}}$
$=-5\times-10=-50$
$\text{R.H.S}=\sqrt[3]{-125}\times\sqrt[3]{-1000}$
$=\sqrt[3]{-5\times-5\times-5}\times\sqrt[3]{{\{-10\times-10\times-10\}}}$
$=-5\times-10=50$
Because LHS is equal to RHS, the equation is true.
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Question 92 Marks
Using the method of successive subtraction examine whether or not the following numbers are perfect cubes:
1331
Answer
Applying subtraction method, We have,
1331 - 1 = 1330
1330 - 7 = 1323
1323 - 19 = 1304
1304 - 37 = 1267
1267 - 61 = 1206
1206 - 91 = 1115
1115 - 127 = 988
988 - 169 = 819
819 - 217 = 602
602 - 271 = 331
331 - 331 = 0
$\because$ Subtraction is performed 11 times.
Hence, 1331 is a perfect cube
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Question 102 Marks
Find the volume of a cube whose surface area is $384 m^2$.
Answer
Surface area of a cube is given by:
$SA =6 s^2$, where $s=$ Side of the cube
Further, volume of a cube is given by:
$V = s ^3$, where $s =$ Side of the cube
It is given that the surface area of the cube is $384 m^2$. Therefore, we have:
$6 s^2=384$
$\Rightarrow s=\sqrt{\frac{384}{6}}$
$=\sqrt{64}=8 m$
Now, volume is given by:
$V=s^3=8^3$
$\Rightarrow V=8 \times 8 \times 8=512 m^3$
Thus, the required volume is $512 m^3$.
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Question 112 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?243000
Answer
On factorising 243000 into prime factors, we get:
243000 = 2 × 2 × 2 × 3 × 3 × 3 × 3 × 3 × 5 × 5 × 5
On grouping the factors in triples of equal factors, we get:
243000 = {2 × 2 × 2} × {3 × 3 × 3} × 3 × 3 × {5 × 5 × 5}
It is evident that the prime factors of 243000 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 243000 is a not perfect cube. However, if the number is divided by (3 × 3 = 9), the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 243000 should be divided by 9 to make it a perfect cube.
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Question 122 Marks
Find the cube root of the following rational numbers:
$\frac{-125}{729}$
Answer
Let us consider the following rational number:
$\frac{-125}{729}$
Now
$\sqrt[3]\frac{-125}{729}$
$=\frac{\sqrt[3]{-125}}{\sqrt[3]{729}}$ $\Big(\because\sqrt[3]{\frac{\text{a}}{\text{}b}}=\sqrt[3]{\frac{\sqrt[3]{\text{a}}}{\sqrt[3]{\text{b}}}}\Big)$
$=\frac{-\sqrt[3]{125}}{\sqrt[3]{729}}$ $\Big(\because\sqrt[3]{\text{-a}}=-\sqrt[3]{a}\Big)$
$=-\frac{5}{9}$ $\Big(\because$ 9 × 9 × 9 and 125 = 5 × 5 × 5$\Big)$
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Question 132 Marks
Which of the following number are not perfect cube?
1728
Answer
On factorising 1728 into prime factors, we get:
1728 = 2 × 2 × 2 × 2 × 2 × 2 × 3 × 3 × 3
On grouping the factors in triples of equal factors, we get:
1728 = {2 × 2 × 2} × {2 × 2 × 2} × {3 × 3 × 3}
It is evident that the prime factors of 1728 can be grouped into triples of equal factors and no factor is left over. Therefore, 1728 is a perfect cube.
Thus, (iii) 243 is the required number, which is not a perfect cube.
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Question 142 Marks
What is the smallest number by which the following number must be multiplied, so that the products are perfect cube?
1323
Answer
On factorising 675 into prime factors, we get:
1323 = 3 × 3 × 3 × 7 × 7
On grouping the factors in triples of equal factors, we get:
1323 = {3 × 3 × 3} × 5 × 5
It is evident that the prime factors of 1323 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 1323 is a not perfect cube. However, if the number is multiplied by 7, the factors can be grouped into triples of equal factors and no factor will be left over.
Thus, 1323 should be multiplied by 7 to make it a perfect cube.
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Question 152 Marks
Making use of the cube root table, find the cube root 0.27
Answer
The number 0.27 can be written as $\frac{27}{100}.$
Now
$\sqrt[3]{0.27}$
$=\sqrt[3]{\frac{27}{100}}$
$={\frac{\sqrt[3]{27}}{\sqrt[3]{100}}}$
$=\frac{3}{\sqrt[3]{100}}$
By cube root table, we have:
$\sqrt[3]{100}=4.642$
$\therefore\sqrt[3]{0.27}$
$=\frac{3}{\sqrt[3]{100}}$
$=\frac{3}{4.642}$
$=0.646$
Thus, the required cube root is 0.646.
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Question 162 Marks
Making use of the cube root table, find the cube root 250
Answer
We have:
250 = 25 × 100
$\therefore$ Cube root of 250 would be in the column of $\sqrt[3]{10\text{x}}$ against 25.
By the cube root table, we have:
$\sqrt[3]{250}=6.3$
Thus, the required cube root is 6.3.
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Question 172 Marks
Which of the following are perfect cube?
243
Answer
On factorising 243 into prime factors, we get
243 = 3 × 3 × 3 × 3 × 3
Group the factors in triples of equal factors as:
243 = {3 × 3 × 3} × 3 × 3
It is evident that the prime factors of 243 can be grouped into triples of equal factors and no factor is left over.
Therefore, 243 is a perfect cube.
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Question 182 Marks
What happens to the cube of a number if the number is multiplied by.
5?
Answer
Let us consider a number $n$. Its cube would be $n^3$. If $n$ is multiplied by 5 , it becomes $5 n$.
Let us now find the cube of $4 n$, we get:
$(5 n)^3=5^3 \times n^3=125 n^3$
Therefore, the cube of $5 n$ is 125 times of the cube of $n$.
Thus, if a number is multiplied by 5 , its cube is 125 times of the cube of that number.
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Question 192 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?8788
Answer
On factorising 8788 into prime factors, we get:
8788 = 2 × 2 × 13 × 13 × 13
On grouping the factors in triples of equal factors, we get:
8788 = 2 × 2 × {13 × 13 × 13}
It is evident that the prime factors of 8788 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 8788 is a not perfect cube. However, if the number is divided by (2 × 2 = 42 × 2 = 4), the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 8788 should be divided by 4 to make it a perfect cube.
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Question 202 Marks
Making use of the cube root table, find the cube root 8.6
Answer
The number 8.6 can be written as $\frac{86}{10}.$
Now
$\sqrt[3]{8.6}$
$=\sqrt[3]{\frac{86}{100}}$
$={\frac{\sqrt[3]{86}}{\sqrt[3]{10}}}$
By cube root table, we have:
$\sqrt[3]{86}=4.414$ and $\sqrt[3]{10}=2.154$
$\therefore\sqrt[3]{8.6}$
$=\frac{\sqrt[3]{86}}{\sqrt[3]{10}}$
$=\frac{4.414}{2.154}$
$=2.049$
Thus, the required cube root is 2.049.
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Question 212 Marks
Find the cube roots of the following numbers by successive subtraction of numbers:
1, 7, 19, 37, 61, 91, 127, 169, 217, 271, 331, 397, ...
64
Answer
We have,
64 - 1 = 63
63 - 7 = 56
56 - 19 =37
37 - 37 = 0
$\because$ Subtraction is performed 4 times.
Hence, cube root of 64 is 4.
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Question 222 Marks
Show that:
$\sqrt[3]{27}\times\sqrt[3]{64}=\sqrt[3]{27\times64}$
Answer
$\text{L.H.S}=\sqrt[3]{27}\times\sqrt[3]{64}$
$\sqrt[3]{27\times64}$
$=\sqrt[3]{3\times3\times3}\times\sqrt[3]{4\times4\times4}$
$=3\times4=12$
$\text{R.H.S}=\sqrt[3]{27}\times\sqrt[3]{64}$
$=\sqrt[3]{3\times3\times3\times4\times4\times4}$
$=\sqrt[3]{\{3\times3\times3\times\}\{4\times4\times4\}}$
$=3\times4=12$
Because LHS is equal to RHS, the equation is true.
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Question 232 Marks
Making use of the cube root table, find the cube root 70
Answer
Because 70 lies between 1 and 100, we will look at the row containing 70 in the column of x.
By the cube root table, we have:
$\sqrt[3]{70}=4.121$
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Question 242 Marks
Making use of the cube root table, find the cube root 133100
Answer
We have:
133100 = 1331 × 100
$\Rightarrow\sqrt[3]{1331100}$
$=\sqrt[3]{1331\times100} $
$=11\times\sqrt[3]{100}$
By cube root table, we have:
$\sqrt[3]{100}=4.642 $
$\therefore\sqrt[3]{133100}$
$=11\times\sqrt[3]{100}$
$=11\times4.642$
$=51.062$
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Question 252 Marks
What is the smallest number by which the following number must be multiplied, so that the products are perfect cube?
675
Answer
On factorising 675 into prime factors, we get:
675 = 3 × 3 × 3 × 5 × 5
On grouping the factors in triples of equal factors, we get:
675 = {3 × 3 × 3} × 5 × 5
It is evident that the prime factors of 675 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 675 is a not perfect cube. However, if the number is multiplied by 5, the factors can be grouped into triples of equal factors and no factor will be left over.
Thus, 675 should be multiplied by 5 to make it a perfect cube.
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Question 262 Marks
Evaluate the following:
$\sqrt[3]{0.1\times0.1\times0.1\times13\times13\times13}$
Answer
To evaluate the value of the expression, we need to proceed as follows:
$\sqrt[3]{0.1\times0.1\times0.1\times13\times13\times13}$
$=\sqrt[3]{\frac{1}{10}\times\frac{1}{10}\times\frac{1}{10}\times13\times13\times13}$
$=\sqrt[3]{\frac{13\times13\times13}{10\times10\times10}}$
$=\frac{\sqrt[3]{13\times13\times13}}{\sqrt[3]{10\times10\times10}}$
$=\frac{13}{10}=1.3$
Thus, the answer is 1.3.
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Question 272 Marks
Which of the following are perfect cube?3087
Answer
On factorising 3087 into prime factors, we get
3087 = 3 × 3 × 7 × 7 × 7
Group the factors in triples of equal factors as:
3087 = 3 × 3 × {7 × 7 × 7}
It is evident that the prime factors of 3087 can be grouped into triples of equal factors and no factor is left over.
Therefore, 243 is a perfect cube.
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Question 282 Marks
What is the smallest number by which the following number must be multiplied, so that the products are perfect cube?
2560
Answer
On factorising 2560 into prime factors, we get:
2560 = 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 5
On grouping the factors in triples of equal factors, we get:
2560 = {2 × 2 × 2} × {2 × 2 × 2} × {2 × 2 × 2} × 5
It is evident that the prime factors of 2560 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 2560 is a not perfect cube. However, if the number is multiplied by 5 × 5 = 255 × 5 = 25, the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 2560 should be multiplied by 25 to make it a perfect cube.
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Question 292 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?
675
Answer
On factorising 675 into prime factors, we get:
675 = 3 × 3 × 3 × 5 × 5
On grouping the factors in triples of equal factors, we get:
675 = {3 × 3 × 3} × 5 × 5
It is evident that the prime factors of 675 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 675 is a not perfect cube. However, if the number is divided by 5 × 5 = 255 × 5 = 25, the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 675 should be divided by 25 to make it a perfect cube.
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Question 302 Marks
Using the method of successive subtraction examine whether or not the following numbers are perfect cubes:
792
Answer
Applying subtraction method, We have,
792 - 1 = 791
791 - 7 = 784
784 - 19 = 765
765 - 37 = 728
728 - 61 = 667
667 - 91 = 576
576 - 127 = 449
449 - 169 = 280
280 - 217 = 63
$\because$ Next number to be subtracted is 271, which is greter than 63
Hence, 792 is not a perfect cube
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Question 312 Marks
Evaluate:
$\sqrt[3]{700\times2\times49\times5}$
Answer
Property:
For any two integers a and b, $\sqrt[3]{\text{ab}}=\sqrt[3]{\text{a}}\times\sqrt[3]{\text{b}},$
From the above property, we have:​
$\sqrt[3]{700\times2\times49\times5}$
$=\sqrt[3]{2\times2\times5\times5\times7\times2\times7\times7\times5}$
$\therefore$ 700 = 2 × 2 × 5 × 5 × 7 and 49 = 7 × 7
$=\sqrt[3]{2^3\times5^3\times7^3}$
$=\sqrt[3]{2^3}\times\sqrt[3]5^3\times\sqrt[3]{7^3}$
$=2\times5\times7=70$
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Question 322 Marks
Making use of the cube root table, find the cube root 0.86
Answer
The number 0.86 can be written as $\frac{86}{100}.$
Now
$\sqrt[3]{0.86}$
$=\sqrt[3]{\frac{86}{100}}$
$={\frac{\sqrt[3]{86}}{\sqrt[3]{100}}}$
By cube root table, we have:
$\sqrt[3]{86}=4.414$ and $\sqrt[3]{100}=4.642$
$\therefore\sqrt[3]{0.86}$
$=\frac{\sqrt[3]{86}}{\sqrt[3]{100}}$
$=\frac{4.414}{4.642}$
$=0.951$ (upto three decimal places)
Thus, the required cube root is 0.951.
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Question 332 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?35721
Answer
On factorising 35721 into prime factors, we get:
35721 = 3 × 3 × 3 × 3 × 3 × 3 × 7 × 7
On grouping the factors in triples of equal factors, we get:
35721 = {3 × 3 × 3} × {3 × 3 × 3} × 7 × 7
It is evident that the prime factors of 35721 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 35721 is a not perfect cube. However, if the number is divided by (7 × 7 = 497), the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 35721 should be divided by 49 to make it a perfect cube.
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Question 342 Marks
Evaluate the following:
$\sqrt[3]{\frac{0.027}{0.008}}\div\sqrt\frac{0.09}{0.04}-1$
Answer
To evaluate the value of the given expression, we need to proceed as follows: $\sqrt[3]{\frac{0.027}{0.008}}\div\sqrt\frac{0.09}{0.04}-1$ $=\sqrt[3]{\frac{\frac{27}{1000}}{\frac{8}{1000}}}\div\sqrt[3]{\frac{\frac{9}{100}}{\frac{4}{100}}}-1$ $=\sqrt[3]{\frac{27}{8}}\div\sqrt[3]{\frac{9}{4}}-1$ $={\frac{\sqrt[3]{27}}{\sqrt[3]{8}}}\div{\frac{\sqrt[3]{9}}{\sqrt[3]{4}}}-1$$=\frac{3}{2}\div\frac{3}{2}-1$
$= \frac{3^1}{2}\times\frac{2^1}{3}-1$
$=1-1=0$
Thus, the answer is 0.
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Question 352 Marks
Show that:
$\frac{\sqrt[3]{729}}{\sqrt[3]{1000}}=\frac{\sqrt[3]{729}}{\sqrt[3]{1000}}$
Answer
$\text{LHS}=\frac{\sqrt[3]{729}}{\sqrt[3]{1000}}$
$=\frac{\sqrt[3]{9\times9\times9}}{\sqrt[3]{10\times10\times10}}$
$=\frac{9}{10}$
$\text{RHS}=\sqrt[3]\frac{{729}}{{1000}}$
$=\sqrt[3]\frac{{9\times9\times9}}{{10\times10\times10}}$
$=\sqrt[3]{\frac{9}{10}\times\frac{9}{10}\times\frac{9}{10}}$
$=\sqrt[3]{(\frac{9}{10})^3}$
$=\frac{9}{10}$
Because LHS is equal to RHS, the equation is true.
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Question 362 Marks
Show that:
$\sqrt[3]{64}\times\sqrt[3]{729}=\sqrt[3]{64\times729}$
Answer
$\text{L.H.S}=\sqrt[3]{64\times729}$
$=\sqrt[3]{4\times4\times4\times9\times9\times9}$
$=\sqrt[3]{\{4\times4\times4\}\times\{9\times9\times9\}}$
$=4\times9=36$
$\text{R.H.S}=\sqrt[3]{64}\times\sqrt[3]{729}$
$=\sqrt[3]{4\times4\times4}\times\sqrt[3]{9\times9\times9}$
$=4\times9=36$
Because LHS is equal to RHS, the equation is true.
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Question 372 Marks
Using the method of successive subtraction examine whether or not the following numbers are perfect cubes:
130
Answer
Applying subtraction method, We have,
130 - 1 = 129
129 - 7 = 122
122 - 19 = 103
103 - 37 = 66
66 - 61 = 5
$\because$ Next number to be subtracted is 91, which is greter than 5
Hence, 130 is not a perfect cube.
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Question 382 Marks
Which of the following are perfect cube?1728
Answer
On factorising 1728 into prime factors, we get
1728 = 2 × 2 × 2 × 2 × 2 × 2 × 3 × 3 × 3
Group the factors in triples of equal factors as:
1728 = {2 × 2 × 2} × {2 × 2 × 2} × {3 × 3 × 3}
It is evident that the prime factors of 1728 can be grouped into triples of equal factors and no factor is left over.
Therefore, 1728 is a perfect cube.
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Question 392 Marks
What happens to the cube of a number if the number is multiplied by.
3?
Answer
Let us consider a number $n$. Its cube would be $n^3$. If $n$ is multiplied by 3 , it becomes $3 n$.
Let us now find the cube of $3 n$, we get:
$(3 n)^3=3^3 \times n^3=27 n^3$
Therefore, the cube of $3 n$ is 27 times of the cube of $n$.
Thus, if a number is multiplied by 3 , its cube is 27 times of the cube of that number.
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Question 402 Marks
Which of the following are perfect cube?
216
Answer
On factorising 216 into prime factors, we get
216 = 2 × 2 × 2 × 3 × 3 × 3
Group the factors in triples of equal factors as:
216 = {2 × 2 × 2} × {3 × 3 × 3}
It is evident that the prime factors of 216 can be grouped into triples of equal factors and no factor is left over.
Therefore, 216 is a perfect cube.
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Question 412 Marks
Which of the following number are not perfect cube?
243
Answer
On factorising 243 into prime factors, we get:
243 = 3 × 3 × 3 × 3 × 3
On grouping the factors in triples of equal factors, we get:
243 = {3 × 3 × 3} × 3 × 3
It is evident that the prime factors of 243 can be grouped into triples of equal factors and no factor is left over. Therefore, 243 is a perfect cube.
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Question 422 Marks
What is the smallest number by which the following number must be multiplied, so that the product are perfects cube?
357221
Answer
On factorising 35721 into prime factors, we get:
35721 = 3 × 3 × 3 × 3 × 3 × 3 × 7 × 7
On grouping the factors in triples of equal factors, we get:
35721 = {3 × 3 × 3} × {3 × 3 × 3} × 7 × 7
It is evident that the prime factors of 35721 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 35721 is a not perfect cube. However, if the number is multiplied by 7, the factors be grouped into triples of equal factors such that no factor is left over.
Thus, 35721 should be multiplied by 7 to make it a perfect cube.
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Question 432 Marks
Which of the following number are not perfect cube?
64
Answer
On factorising 64 into prime factors, we get:
64 = 2 × 2 × 2 × 2 × 2 × 2
On grouping the factors in triples of equal factors, we get:
64 = {2 × 2 × 2} × {2 × 2 × 2}
It is evident that the prime factors of 64 can be grouped into triples of equal factors and no factor is left over. Therefore, 64 is a perfect cube.
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Question 442 Marks
Which of the following are perfect cube?166375
Answer
On factorising 166375 into prime factors, we get
166375 = 5 × 5 × 5 × 11 × 11 × 11
Group the factors in triples of equal factors as:
166375 = {5 × 5 × 5} × {11 × 11 × 11}
It is evident that the prime factors of 166375 can be grouped into triples of equal factors and no factor is left over.
Therefore, 166375 is a perfect cube.
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Question 452 Marks
Making use of the cube root table, find the cube root 700
Answer
We have:
700 = 70 × 10
$\therefore$ Cube root of 700 will be in the column of $\sqrt[3]{10\text{x}}$ against 70.
By the cube root table, we have:
$\sqrt[3]{700}=8.879$
Thus, the answer is 8.879.
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Question 462 Marks
What happens to the cube of a number if the number is multiplied by.
4?
Answer
Let us consider a number $n$. Its cube would be $n ^3$. If n is multiplied by 4 , it becomes $4 n$.
Let us now find the cube of $4 n$, we get:
$(4 n)^3=4^3 \times n^3=64 n^3$
Therefore, the cube of $4 n$ is 64 times of the cube of $n$.
Thus, if a number is multiplied by 4 , its cube is 64 times of the cube of that number.
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Question 472 Marks
Which of the following are perfect cube?
64
Answer
On factorising 64 into prime factors, we get
64 = 2 × 2 × 2 × 2 × 2 × 2
Group the factors in triples of equal factors as:
64 = {2 × 2 × 2} × {2 × 2 × 2}
It is evident that the prime factors of 64 can be grouped into triples of equal factors and no factor is left over.
Therefore, 64 is a perfect cube.
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Question 482 Marks
By which smallest number must the following numbers be divided so that the quotient is a perfect cube?107811
Answer
On factorising 107811 into prime factors, we get:
107811 = 3 × 3 × 3 × 3 × 11 × 11 × 11
On grouping the factors in triples of equal factors, we get:
107811 = {3 × 3 × 3} × 3 × {11 × 11 × 11}
It is evident that the prime factors of 107811 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 107811 is a not perfect cube. However, if the number is divided by 3, the factors can be grouped into triples of equal factors such that no factor is left over.
Thus, 107811 should be divided by 3 to make it a perfect cube.
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Question 492 Marks
Using the method of successive subtraction examine whether or not the following numbers are perfect cubes:
345
Answer
Applying subtraction method, We have,
345 - 1 = 344
344 - 7 = 337
337 - 19 = 318
318 - 37 = 281
281 - 61 = 220
220 - 91 = 129
129 - 127 = 2
$\because$ Next number to be subtracted is 169, which is greter than 2
Hence, 345 is not a perfect cube
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Question 502 Marks
What is the smallest number by which the following number must be multiplied, so that the product are perfects cube?
107811
Answer
On factorising 107811 into prime factors, we get:
107811 = 3 × 3 × 3 × 3 × 11 × 11 × 11
On grouping the factors in triples of equal factors, we get:
107811 = {3 × 3 × 3} × 3 × {11 × 11 × 11}
It is evident that the prime factors of 107811 cannot be grouped into triples of equal factors such that no factor is left over. Therefore, 107811 is a not perfect cube. However, if the number is multiplied by 3 × 3 = 93 × 3 = 9, the factors be grouped into triples of equal factors such that no factor is left over.
Thus, 107811 should be multiplied by 9 to make it a perfect cube.
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