Question

12th term of an A.P is - 13 and sum of first four terms is 24 find the sum of first 20 terms

Answer 1

Answer:

a12=-13

s4=24

s20=?

-13=a+11d

• -13-11d=a

24=4/2(2(-13-11d)+3d)

24/2=-26-22d+3d

12=-26-19d

12+26=-19d

38=-19d

d=-2

-13-11d=a

-13-11(-2)=a

-13+22=a

9=a

s20=20/2(2×9+19×-2)

s20=10(18-38)

s20=-200