Let a1, a2, a3, a4 …….. be an arithmetic progression and g1, g2, g3, g4 …… be a geometric progression. If a1+g1=1, a2+g2=4, a3+g3=15 and a4+g4=2, then
the common ratio of geometric progression is equal to –3
Let a and d be first term and common difference of A.P. Also, let b and r be the first term and common ratio of G.P.
∴ On solving, we get
a=12, b=12, d=5, r=−3Also, ∑20k=1ak=960