${7^{th}}$ term of an $A.P.$ is $40$, then the sum of first $13$ terms is
${7^{th}}$ term of an $A.P.$ is $40$, then the sum of first $13$ terms is
- A
$53$
- B
$520$
- C
$1040$
- D
$2080$
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- [KVPY 2014]
Sum of the first $p, q$ and $r$ terms of an $A.P.$ are $a, b$ and $c,$ respectively. Prove that $\frac{a}{p}(q-r)+\frac{b}{q}(r-p)+\frac{c}{r}(p-q)=0$
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Write the first five terms of the following sequence and obtain the corresponding series :
$a_{1}=a_{2}=2, a_{n}=a_{n-1}-1, n\,>\,2$
Write the first five terms of the following sequence and obtain the corresponding series :
$a_{1}=a_{2}=2, a_{n}=a_{n-1}-1, n\,>\,2$