Gujarati
Hindi
8. Sequences and Series
normal

Let $V_{\mathrm{r}}$ denote the sum of the first $\mathrm{r}$ terms of an arithmetic progression $(A.P.)$ whose first term is $\mathrm{r}$ and the common difference is $(2 \mathrm{r}-1)$. Let

$T_{\mathrm{I}}=V_{\mathrm{r}+1}-V_{\mathrm{I}}-2 \text { and } \mathrm{Q}_{\mathrm{I}}=T_{\mathrm{r}+1}-\mathrm{T}_{\mathrm{r}} \text { for } \mathrm{r}=1,2, \ldots$

$1.$  The sum $V_1+V_2+\ldots+V_n$ is

$(A)$ $\frac{1}{12} n(n+1)\left(3 n^2-n+1\right)$

$(B)$ $\frac{1}{12} n(n+1)\left(3 n^2+n+2\right)$

$(C)$ $\frac{1}{2} n\left(2 n^2-n+1\right)$

$(D)$ $\frac{1}{3}\left(2 n^3-2 n+3\right)$

$2.$  $\mathrm{T}_{\mathrm{T}}$ is always

$(A)$ an odd number $(B)$ an even number

$(C)$ a prime number $(D)$ a composite number

$3.$  Which one of the following is a correct statement?

$(A)$ $Q_1, Q_2, Q_3, \ldots$ are in $A.P.$ with common difference $5$

$(B)$ $\mathrm{Q}_1, \mathrm{Q}_2, \mathrm{Q}_3, \ldots$ are in $A.P.$ with common difference $6$

$(C)$ $\mathrm{Q}_1, \mathrm{Q}_2, \mathrm{Q}_3, \ldots$ are in $A.P.$ with common difference $11$

$(D)$ $Q_1=Q_2=Q_3=\ldots$

Give the answer question $1,2$ and $3.$

A

$B,D,B$

B

$B,D,A$

C

$A,C,B$

D

$D,B,A$

(IIT-2007)

Solution

$1.$ $ \mathrm{V}_{\mathrm{r}}=\frac{\mathrm{r}}{2}[2 \mathrm{r}+(\mathrm{r}-1)(2 \mathrm{r}-1)]=\frac{1}{2}\left(2 \mathrm{r}^3-\mathrm{r}^2+\mathrm{r}\right)$

$ \sum \mathrm{V}_{\mathrm{r}}=\frac{1}{12} \mathrm{n}(\mathrm{n}+1)\left(3 \mathrm{n}^2+\mathrm{n}+2\right)$

$2.$ $ \mathrm{V}_{\mathrm{r}+1}-\mathrm{V}_{\mathrm{r}}=(\mathrm{r}+1)^3-\mathrm{r}^3-\frac{1}{2}\left[(\mathrm{r}+1)^2-\mathrm{r}^2\right]+\frac{1}{2} $

$ =3 \mathrm{r}^2+2 \mathrm{r}+1 $

$ \mathrm{~T}_{\mathrm{r}}=3 \mathrm{r}^2+2 \mathrm{r}-1=(\mathrm{r}+1)(3 \mathrm{r}-1)$

which is a composite number.

$3.$  $ \mathrm{T}_{\mathrm{r}}=3 \mathrm{r}^2+2 \mathrm{r}-1 $

$ \mathrm{~T}_{\mathrm{r}+1}=3(\mathrm{r}+1)^2+2(\mathrm{r}+1)-1 $

$ \mathrm{Q}_{\mathrm{r}}=\mathrm{T}_{\mathrm{r}+1}-\mathrm{T}_{\mathrm{r}}=3[2 \mathrm{r}+1]+2[1] $

$ \mathrm{Q}_{\mathrm{r}}=6 \mathrm{r}+5 $

$ \mathrm{Q}_{\mathrm{r}+1}=6(\mathrm{r}+1)+5$

Common difference $=\mathrm{Q}_{\mathrm{r}+1}-\mathrm{Q}_{\mathrm{I}}=6$.

Standard 11
Mathematics

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