Question

7th term of an AP is 40.Then the sum of its first 13th term is

Answer 1

Answer:

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

a+(7−1)d=40

a+6d=40

Now sum of 13 term is

13/2 (2a+12d)= 520

.

Answer 2

Answer:

7th term=40=a+6b

here a is the 1st term and b is the common difference

sum of its first 13th term is

13/2(2a+12b)

=13/2.2(a+6b)

=13. 40=520