Question

for an AP the 12th term is -13 and the sum of its first 4terms is 24. find the sum of first 10 terms of the AP.

Answer 1

Let

a=first term

d=common difference

a+(12-1)d=-13

a+11d=-13.................(1)

S=n/2*[2a+(n-1)d]

24=4/2*[2a+3d]

2a+3d=12..................(2)

Multiplying (1) by 2 we get:

2a+22d=-26..............(3)

Subtracting (2) and (3) we get:-

-19d=38

d=-38/19

=-2.

2a+3d=12

=2a-6=12

=2a=18

a=18/2=9.

S=n/2*[2a+(n-1)d]

S=10/2*[18+9*-2]

=10/2*[18-18]

=10/2*0

=0

Hope it helps you.