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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 $3.$
$B,D,B$
$B,D,A$
$A,C,B$
$D,B,A$
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 $=\mathrm{Q}_{\mathrm{r}+1}-\mathrm{Q}_{\mathrm{I}}=6$.