Question

Find the sum of first 20 terms ofan ap whose 3rd term is 10 and when 6th term is subtracted from 9th term we get 24

Answer 1

T3=10. T9-T6=24
a+8d-(a+5d)=24
a+8d-a-5d=24
3d=24
d=8
T3=10
a+2d=10
a+2(8)=10
a=10-16
a=-6

sn=n/2 (2a+(n-1)d

20/2 (2(-6)+(20-1)×8
10 (-12+152)
10×140
1400

sum of first 20 terms are 1400