1 Marks Question

Question 11 Mark

Write the sum to n terms of a series whose r^th term is r + 2^r

Answer

Let T_n be the n^th term of the series and S_n be the sum to n terms of a series.
$\therefore\ \text{T}_\text{r}=\text{r}+2^{\text{r}}$ [given]
$\therefore\ \text{S}_\text{n}=\sum\limits^{\text{n}}_{\text{r}=1}(\text{r}+2^{\text{r}})$
$=\sum\limits^{\text{n}}_{\text{r}=1}\text{r}+\sum\limits^{\text{n}}_{\text{r}=1}2^{\text{r}}$
$=\frac{\text{n}(\text{n}+1)}{2}+\big[2^1+2^2\ ...\ 2^{\text{n}}\big]$
$=\frac{\text{n}(\text{n}+1)}{2}+\frac{2(2^\text{n}-1)}{(2-1)}$
$=\frac{\text{n}(\text{n}+1)}{2}+\frac{2\times2^\text{n}-2}{1}$
$=\frac{\text{n}(\text{n}+1)}{2}+2^{\text{n}+1}-2$
$\therefore\ \text{S}_\text{n}=\frac{\text{n}(\text{n}+1)}{2}+2^{\text{n}+1}-2$

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Question 21 Mark

Write the sum of the series 1² - 2² + 3² - 4² + 5² - 6² + ..... + (2n - 1)² - (2n)²

Answer

We have,
1² - 2² + 3² - 4² + 5² - 6² + ..... + (2n - 1)² - (2n)²
= (1 - 2)(1 + 2) + (3 - 4)(3 + 4) + ...... + (2n - 1 - 2n)(2n - 1 + 2n)
= -1[1 + 2 + 3 + 4 + ..... + 2n - 1 + 2n]
$=-1\Big[\frac{2\text{n}(2\text{n}+1)}{2}\Big]$ $\Big[\because\ 1 + 2\ ....\text{n} =\frac{\text{n}(\text{n}+1)}{2}\Big]$
$=-\text{n}(2\text{n}+1)$
Hence, sum of the series = -n(2n + 1)

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Question 31 Mark

If the sum of first n even natural numbers is equal to k times the sum of first n odd natural numbers, then write the value of k.

Answer

It is given that the sum of first n even natural numbers is equal to k times the sum of first n odd natural numbers.
$\therefore\ 2+4+6+ \ ...\ +2\text{n}=\text{k}(1+3+5+\ ....\ +\text{n})$
$\Rightarrow2(1+2+3+ \ ...\ +\text{n})=\text{k}(1+3+5+\ ....\ +\text{n})$
$\Rightarrow2\times\frac{\text{n}(\text{n}+1)}{2}=\text{k}(1+3+5+\ ...\ +\text{n})$
$\Rightarrow\text{n}(\text{n}+1)=\text{k}(1+3+5+\ ...\ +\text{n})$
$\Rightarrow\text{n}(\text{n}+1)=\text{k}\times\text{n}^2$ $\big[\because$ sum of first odd natural numbers is n²$\big]$
$\Rightarrow\frac{(\text{n}+1)}{\text{n}}=\text{k}$
Hence, $\text{k}=\frac{\text{n}+1}{\text{n}}$

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Question 41 Mark

Let S_n denote the sum of the cubes of first n natural numbers and s_n denote the sum of first n natural numbers. Then, write the value of $\sum\limits^\text{n}_{\text{r}=1}\frac{\text{S}_\text{r}}{\text{S}_\text{r}}$

Answer

We know that,
$\text{S}_\text{r}=1^3+2^3+3^3+\ ....\ +\text{r}^3=\Big[\frac{\text{r}(\text{r}+1)}{2}\Big]^2$
And, $\text{S}_\text{r}=1+2+3+\ ....\ +\text{r}=\frac{\text{r}(\text{r}+1)}{2}$
As, $\frac{\text{S}_\text{r}}{\text{S}_\text{r}}=\frac{\big[\frac{\text{r}(\text{r}+1)}{2}\big]^2}{\big[\frac{\text{r}(\text{r}+1)}{2}\big]}$
$=\frac{\text{r}(\text{r}+1)}{2}=\frac{1}{2}(\text{r}^2+\text{r})$
Now,
$\sum\limits^{\text{n}}_{\text{r}=1}\frac{\text{S}_\text{r}}{\text{S}_\text{r}}=\sum\limits^{\text{n}}_{\text{r}=1}\frac{1}{2}(\text{r}^2+\text{r})$
$=\frac{1}{2}\Bigg(\sum\limits^{\text{n}}_{\text{r}=1}\text{r}^2+\sum\limits^{\text{n}}_{\text{r}=1}\text{r}\Bigg)$
$=\frac{1}{2}\Big[\frac{\text{n}(\text{n}+1)(2\text{n}+1)}{6}+\frac{\text{n}(\text{n}+1)}{2}\Big]$
$=\frac{1}{2}\times\frac{\text{n}(\text{n}+1)}{2}\times\Big[\frac{(2\text{n}+1)}{3}+1\Big]$
$=\frac{\text{n}(\text{n}+1)}{4}\Big[\frac{2\text{n}+4}{3}\Big]$
$=\frac{\text{n}(\text{n}+1)}{4}\times\frac{2(\text{n}+2)}{3}$
$=\frac{\text{n}(\text{n}+1)(\text{n}+2)}{6}$

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Question 51 Mark

If $\sum\limits^{\text{n}}_{\text{r}=1}\text{r}=55,$ find $\sum\limits^{\text{n}}_{\text{r}=1}\text{r}^3$

Answer

$\sum\limits^{\text{n}}_{\text{r}=1}\text{r}=55$
$\Rightarrow\frac{\text{n}(\text{n}+1)}{2}=55\ ...(\text{i})$
Now,
$\sum\limits^{\text{n}}_{\text{r}=1}\text{r}^3=\Big[\frac{\text{n}(\text{n}+1)}{2}\Big]^2$
$=\big[55\big]^2$ [Using equation (i), we get]
$=3025$
Hence, $\sum\limits^{\text{n}}_{\text{r}=1}\text{r}^3=3025$

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Question 61 Mark

Write the sum of the series 2 + 4 + 6 + 8 + ... + 2n.

Answer

Let T_n be the term of the given series and S_n be the sum of the given series.
$\therefore\ \text{T}_\text{n}=2\text{n}$
$\therefore\ \text{S}_\text{n}=\sum\limits^{\text{n}}_{\text{k}=1}\text{T}_\text{k}=\sum\limits^{\text{n}}_{\text{k}=1}2\text{k}$
$=2\sum\limits^{\text{n}}_{\text{k}=1}\text{k}$
$=2\Big[\frac{\text{n}(\text{n}+1)}{2}\Big]$
$=\text{n}(\text{n}+1)$
Hence, $\text{S}_\text{n}=\text{n}(\text{n}+1)$

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Question 71 Mark

Write the sum of 20 terms of the series $1+ \frac{1}{2}(1+2)+\frac{1}{3}(1+2+3)+\ ...$

Answer

Let the n^th term of the given series is T_n and S_n be the sum of the given series.
$\therefore\ \text{T}_\text{n}=\frac{1}{\text{n}}\big[1+2+3+\ ...\ +\text{n}]$
$=\frac{1}{\text{n}}\Big[\frac{\text{n}}{2}\big(2\times1+(\text{n}-1)\times1\big)\Big]$
$=\frac{1}{\text{n}}\times\frac{\text{n}}{2}\big[2+\text{n}-1\big]$
$=\frac{1}{2}(\text{n}+1)$
$\therefore\ \text{S}_\text{n}=\sum\limits^{\text{n}}_{\text{k}=1}\frac{1}{2}(\text{k}+1)$
$=\frac{1}{2}\sum\limits^{\text{n}}_{\text{k}=1}\text{k}+\frac{1}{2}\sum\limits^{\text{n}}_{\text{k}=1}1$
$=\frac{1}{2}\Big[\frac{\text{n}(\text{n}+1)}{2}\Big]+\frac{1}{2}\times\text{n}$
$=\frac{\text{n}(\text{n}+1)}{4}+\frac{\text{n}}{2}$
$=\frac{\text{n}(\text{n}+1)+2\text{n}}{4}$
$=\frac{\text{n}[\text{n}+1+2]}{4}$
$=\frac{\text{n}[\text{n}+3]}{4}$
Putting, n = 20 we get
$\text{S}_{20}=\frac{20[20+3]}{4}$
$=5\times23$
$=115$

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Question 81 Mark

Write the 50^th term of the series 2 + 3 + 6 + 11 + 18 + ....

Answer

We have,
a₁ = 2,
a₂ = 3 = 2 + 1,
a₃ = 6 = 2 + 1 + 3,
a₄ = 11 = 2 + 1 + 3 + 5,
.............................
.............................
..............................
a₅₀ = 2 + 1 + 3 + 5 + .... (50 terms)
$=2+\frac{49}{2}\big[2\times1+(49-1)\times2\big]$ (As, the terms apart 2 are in A.P. with a = 1 and d = 2)
$=2+\frac{49}{2}(2+48\times2)$
$=2+\frac{49}{2}\times98$
$=2+49^2$
$=2+2401$
$=2403$
So, the 50^th term of the given series is 2403

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