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Question 11 Mark
Find out the following sentences are statements and which are not. Justify your answer.
All triangles have three sides.
Answer
It is a true declarative sentence because a figure that has three sides is a triangle. Thus, it is a true statement.
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Question 21 Mark
There is a complex number which is not a real number.
Answer
Negation of the given statement:
All complex numbers are real numbers.
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Question 31 Mark
Write the following statements in the form "if p, then q".
Whenever it rains it is cold.
Answer
If it rains, then it is cold.
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Question 41 Mark
Write the negation of the following statements:
p: For every positive real number x, the number (x - 1) is also positive.
Answer
P: For every positive number x, the number (x - 1) is also positive.
P: At least for one positive real number x, the number (x - 1) is not positive.
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Question 51 Mark
Check whether the following statement are true or not:
p: If x and y are odd integers, then x + y is an even integer.
Answer
Let q and r be the statements given by
q: x and y are odd integers.
r: x + y is an even integer.
Then, the given statement is
if q, then r,
Direct Metflod: Let q be true. Then,
q is true.
⇒ x and y are odd integers
⇒ x = 2m + 1, y = 2n + 1 for some integers m, n
⇒ x + y = (2m + 1) + (2n + 1)
⇒ x + y = (2m + 2n + 2)
⇒ x + y = 2 (m + n + 1)
⇒ x + y is an even integer
⇒ r is true.
Thus, q is true ⇒ r is true.
Hence, ''if q, then r'' is a true statement.
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Question 61 Mark
By giving a counter example, show that the following statement is not true.
p: "If all the angles of a triangle are equal, then the triangle is an obtuse angled triangle".
Answer
Consider a triangle ABC with all angles equal. Then each angle of the triangle is equal to 60".
Hence, ABC is not an obtuse angle triangle.
Therefore the following statement is false.
p: "if all the angles of a triangle are equal, then the triangle is an obtuse angled triangle".
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Question 71 Mark
Show that the statement:
$p$ : "If $x$ is a real number such that $x ^3+ x =0$, then $x$ is $0$ " is true by.
Direct method.
Answer
Let $q$ and $r$ be the statements given
$q : x$ is a real number such that $x ^3+ x =0$.
$r : x$ is $0$ .
Then, p : if q , then r .
Direct metrod: Let $q$ be true. Then,
q is true
$\Rightarrow x$ is a real num bar such that $x^3+x 0$
$\Rightarrow x$ is a real num bar such that $x\left(x^2+1\right)=0$
$\Rightarrow x=0$
$\Rightarrow r$ is true.
Thus, $q$ is true $\Rightarrow r$ is true.
Hence, $p$ is true.
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Question 81 Mark
Find out the following sentences are statements and which are not. Justify your answer.
Is the earth round?
Answer
It is an interrogative sentence, so it is not a statement.
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Question 91 Mark
Determine the contrapositive of the following statements:
If x is less than zero, then x is not positive.
Answer
If x is positive, then x is not less than zero.
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Question 101 Mark
All policemen are thieves.
Answer
Negation of the given statement:
There exists a policeman who is not a thief.
Or
At least one policeman is not a thief.
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Question 111 Mark
Are the following pairs of statements are negation of each other:
The number x is not a rational number.
The number x is an irrational number.
Answer
The statements in this pair are not the negation of each other because both statements are the same. Both the statements convey that x is an irrational number.
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Question 121 Mark
Determine the contrapositive of the following statements:
If Mohan is a poet, then he is poor.
Answer
If Mohan is not poor, then he is not a poet.
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Question 131 Mark
Give three examples of sentences which are not statements. Give reasons for the answers.
Answer
  1. I won the trophy!
It is an exclamatory sentence, so it is not a statement.
  1. Please fetch me a glass of water.
It is an imperative sentence. In other words, it can be expressed either as a request or as a command. Therefore, it not a statement.
  1. Can you do this work for me?
It is an interrogative sentence, so it is not a statement.
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Question 141 Mark
State the converse and contrapositive of the following statements:
If it is hot outside, then you feel thirsty.
Answer
Converse:
If you feel thirsty, then it is hot outside.
Contrapositive:
If you do not feel thirsty, then it is not hot outside.
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Question 151 Mark
Write the following statements in the form "if p, then q".
The game is cancelled only if it is raining.
Answer
If it rains, only then the game is cancelled.
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Question 161 Mark
Determine the contrapositive of the following statements:
It never rains when it is cold.
Answer
If it rains, it is not cold.
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Question 171 Mark
Write the negation of the following statements:
The earth is round.
Answer
Negation of the given statement:
The earth is not round.
Or
It is not true that the earth is round
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Question 181 Mark
Find the component statements of the following compound statements:
25 is a multiple of 5 and 8.
Answer
The component statements of the given compound statement are:
25 is a multiple of 5.
25 is a multiple of 8.
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Question 191 Mark
Find out the following sentences are statements and which are not. Justify your answer.
Listen to me, Ravi.
Answer
It is an exclamatory sentence. Therefore, it is not a statement.
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Question 201 Mark
Check the validity of the following statements:
p: 100 is a multiple of 4 and 5.
Answer
The statem ant is:
"100 ism ultiple of 4 and 5"
We know that 100 is a multiple of 4 as well as 5. So, p is true statement.
Hence, the statement is true i.e. the statement "p" is a valid statement.
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Question 211 Mark
Find the component statements of the following compound statements:
All rational numbers are real and all real numbers are complex.
Answer
The component statements of the given compound statement are:
All rational numbers are real.
All real numbers are complex.
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Question 221 Mark
Determine the contrapositive of the following statements:
It is necessary to be strong in order to be a sailor.
Answer
If you are not strong, then you cannot be a sailor.
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Question 241 Mark
Check whether the following statement are true or not:
q: If x, y are integers such that xy is even, then at least one of x and y is an even integer.
Answer
Let r and s be two statements given by
r: xy is an even integer.
s: At least one of x and y is an even integer
Lets be not true. Then,
s is not true
⇒ Both x and y are odd integers
Let x = 2n + 1 and y = 2m + 1 for some integers n and m. Then,
⇒ xy = (2n + 1)(2m + 1) for some integers n and m.
⇒ xy = 4nm + 2(n + m) + 1 for some integers n and m,
⇒ xy is an odd integer
⇒ xy is not an even integer
⇒ -r is true
Thus, -s is trua es -r is true
Hence, the given statement is true.
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Question 251 Mark
Show that the following statement is true
"The integer $n$ is even if an only if $n^2$ is even"
Answer
The given stat em en $t$ can be re-written as
"The necessary and sufficient condition that the integer $n$ is even is $n 2$ must be even"
Let $p$ and $q$ be the statements given by
p : the integer $n$ is even.
$q : n ^2$ is even.
The given stat em en t is
"p if and only if q"
In order to check its validity, we have to check the validity of the following statements.
i. "If $p$, then $q^{-}$
ii. "if $q$, then $p$ "
Checking the validity of "if $p$, then $q$ ":
The statement "if $p$, then $q^{-}$is given by:
"If the integer $n$ is even, then $n^2$ is even-
Let us assume that $n$ is even. Then,
$n=2 m$, where $m$ is an integer
$\Rightarrow n ^2=(2 m)^2$
$\Rightarrow n ^2=4 m^2$
$\Rightarrow n ^2$ is an even integer
Thus, $n$ is even $\Rightarrow n^2$ is even
$\therefore$ "if $p$, then $q^{-}$is true.
Checking the validity of "if $q$, then p ":
"if $n$ is an integer and $n^2$ is even, 'then $n$ is even"
To check the validity of this statemens, we will use contrapositive method.
So, let $n$ be an odd integer. Then,
n is odd
$\Rightarrow n =2 k +1$ for some integer k :
$\Rightarrow n ^2=(2 k +1)^2$
$\Rightarrow n^2=4 k^2+4 k+1$
$\Rightarrow n ^2$ is not an even integer
Thus, $n$ is not even $\Rightarrow n^2$ is not even
$\therefore$ if $q$, then $p^{-}$is true.
Hence, "p if and only if $q^{-}$is true.
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Question 261 Mark
Write the following statements in the form "if p, then q".
It is necessary to be rich in order to be happy.
Answer
If you want to be happy, then you will have to be rich.
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Question 271 Mark
Write the negation of the following statements:
r: There exists a number x such that 0 < x < 1.
Answer
r: There exists a number x such that 0 < x < 1.
r: For every real number x, either x ≤ 0 or x < 1.
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Question 281 Mark
Find out the following sentences are statements and which are not. Justify your answer.
$\text{x}^2+5|\text{x}|+6=0$ has no real roots.
Answer
It is a true declarative sentence, so it is a statement.
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Question 291 Mark
Find out the following sentences are statements and which are not. Justify your answer.
Mathematics is difficult.
Answer
Mathematics could be easy for some people, so this sentence may or may not be true. So, it is not a statement.
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Question 301 Mark
For the following statements, determine whether an inclusive "OR" or exclusive "OR" is used. Give reasons for your answer.
A lady gives birth to a baby boy or a baby girl.
Answer
Exclusive OR because a lady can give a birth to a baby who is either a boy or a girl.
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Question 311 Mark
Rewrite the following statements in the form "p if and only if q".
s: If a tumbler is half empty, then it is half full and if a tumbler is half full, then it is half empty.
Answer
The tumbler is half empty if and only if the tumbler is half full.
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Question 321 Mark
Find out the following sentences are statements and which are not. Justify your answer.
There are 35 days in a month.
Answer
It is a false assertive sentence, so it is a false statement.
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Question 331 Mark
Check whether the following pair of statements are negation of each other. Give reasons for your answer.
  1. a + b = b + a is true for every real number a and b.
  2. There exist real numbers a and b for which a + b = b + a.
Answer
the negation of ​the statements
a + b = b + a is true for every real number a and b.
is:
There exist real numbers a and b for which a + b ≠ b + a.
So, the given statment is of not tha negation of the first statment.
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Question 341 Mark
Rewrite the following statements in the form "p if and only if q".
r: For you to get an A grade, it is necessary and sufficient that you do all the homework regularly.
Answer
You get an A grade if and only if you do all the homework regularly.
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Question 351 Mark
Which of the following statements are true and which are false? In each case give a valid reason for saying so:
$\text{t}:\sqrt{11}$ is a rational number.
Answer
False. Because square roots of prime num bars are irrational num bars.
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Question 361 Mark
Write the component statements of the following compound statements and check whether the compound statement is true or false:
All rational numbers are real and all real numbers are not complex.
Answer
The component statements of the given compound statement are:
  1. All rational numbers are real.
  2. All real numbers are not complex.
The compound statement is false because all real numbers are complex. The connective used is "and". So, even if one component statement is false, the compound statement is false.
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Question 371 Mark
Write the negation of the following statements:Ravish is honest.
Answer
Negation of the given statement:
It is not true that Ravish is honest.
Or
Ravish is not honest
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Question 381 Mark
Write the negation of the following statements:
Banglore is the capital of Karnataka.
Answer
Negation of the given statement:
It is not true that Bangalore is the capital of Karnataka.
Or
Bangalore is not the capital of Karnataka.
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Question 391 Mark
Both the diagonals of a rectangle have the same length.
Answer
Negation of the given statement:
Both the diagonals of a rectangle do not have the same length.
Or
Both the diagonals of a rectangle have different lengths.
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Question 401 Mark
Write the component statements of the following compound statements and check whether the compound statement is true or false:
The sand heats up quickly in the sun and does not cool down fast at night.
Answer
The component statements of the given compound statement are:
  1. The sand heats up quickly in the sun.
  2. Sand does not cool down fast at night.
The compound statement uses "and" as the connective. For the compound statement to be true, both the component statements must be true. The second component statement "Sand does not cool down fast at night" is false. Sand cools down fast at night. Therefore, the compound statement is false.
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Question 411 Mark
Determine whether the argument used to check the validity of the following statement is correct:
p : "If $x^2$ is irrational, then $x$ is rational"
The statement is true because the number $\text{x}^2=\pi^2$ is irrational, therefore $\text{x}=\pi$ is irrational.
Answer
The argument used to check the validity of the given statement is not correct because it does not produce a contradiction.
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Question 421 Mark
State the converse and contrapositive of the following statements:
If you live in Delhi, then you have winter clothes.
Answer
Converse:
If you have winter clothes, then you live in Delhi.
Contrapositive:
​If you do not have winter clothes, then you do not live in Delhi.
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Question 431 Mark
Which of the following statements are true and which are false? In each case give a valid reason for saying so:
p: Each radius of a circle is a chord of the circle.
Answer
False. Because, no radius of a circle is its chord.
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Question 441 Mark
Find the component statements of the following compound statements:
The sky is blue and the grass is green.
Answer
The component statements of the given compound statement are:
The sky is blue.
The grass is green.
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Question 451 Mark
Negate the following statements:
All the students completed their homework.
Answer
Negation of the given statement:
Some students did not complete their homework.
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Question 461 Mark
Find out the following sentences are statements and which are not. Justify your answer.
Are all circles round?
Answer
It is an interrogative sentence, so it is not a statement.
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Question 471 Mark
Write the following statements in the form "if p, then q".
It never rains when it is cold.
Answer
If it is cold, then it never rains.
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Question 481 Mark
Determine the contrapositive of the following statements:
If Ravish skis, then it snowed.
Answer
If it did not snow, then Ravish does not ski.
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Question 491 Mark
Determine the contrapositive of the following statements:
Only if he does not tire will he win.
Answer
If he tires, then he will not win.
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Question 501 Mark
For the following statements, determine whether an inclusive "OR" or exclusive "OR" is used. Give reasons for your answer.
To entry a country, you need a passport or a voter registration card.
Answer
Inclusive OR is used because a person can have both passport as well as voter registration card.
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