Question

In an AP whose first term is2,the sum of first five terms is one fourth the sum ofbthe next five terms.show that T20=-112.also,finds20

Answer 1

a=2
S5=1/4 (S10-S5)
5/2 (4+4d)=1/4 {[10/2 (4+9d)]-[5/2 (4+4d)]}
5/2 (4+4d=1/4 [5 (4+9d-2-2d)]
8+8d=2+7d
d=-6
T20=2+19×-6
=2-114
=-112