Question

If the 7th term of an A.P.is 40 , then find S13

Answer 1

hope it helps.........

Answer 2

Answer:

Sum of 13 terms is 520.

Step-by-step explanation:

Given: seventh term of an AP = 40

To find: Sum of first 13 terms.

Let, a be the first term and d be the common difference.

We know that,

$a_n=a+(n-1)d$

$a_7=40$

a + ( 7 - 1 )d = 40

a + 6d = 40 ...............(1)

Also,

$S_n=\frac{n}{2}(2a+(n-1)d)$

$S_{13}=\frac{13}{2}(2a+(13-1)d)=\frac{13}{2}(2a+12d)=\frac{13}{2}\times2(a+6d)=13\times40=520$

Therefore, Sum of 13 terms is 520.