Question

In an AP if the 12th term is (-13) and the sun of its first 4 terms us 24. find the sum of its 1st 20 terms.

Answer 1

Answer: -200

Step-by-step explanation:

a+11d=-13      (1)

S4=$\frac{4}{2}$×2a+3d

24=2×2a+3d

2a+3d=12        (2)

Multiply (1)×2 and equate with (2),

2a+22d=-26

2a+3d=12

⇒-19d=38

⇒d=-2

∴a+11(-2)=-13

a-22=-13

a=9

∴S20=10×( 2(9)+19(-2))

=10(18-38)

=10(-20)

=-200