Question

if 12th term of an AP is -13 and the sum of its four terms is 24 , what is the sum of its 10 term

Answer 1

let the terms be a, a+d,...
12th term =-13
a+11d=-13.....................(1)
sum of 4 terms=
s=2(2a+3d)
4a+6d=24...............(2)
solve..
4a+44d=-52...(4*(1))
4a+6d=24
sub :38d=-76
d=-2...a=-13-11(-2)=-13+22=9
therefore sum of 10 terms
s=(n/2)(2a+(n-1)d)
s=10/2(18+9(-2))
s=5(18-18)=5(0)=0