因為是等比數(shù)列,所以去掉奇數(shù)項,或者偶數(shù)項,數(shù)列仍為等比數(shù)列,公比為q^2
故有：
S奇 = 85 = a1(1-q^2n)/(1-q^2) = (1-q^2n)/(1-q^2)
S偶 = 170 = a2(1-q^2n)/(1-q^2)
所以,S偶/S奇 = 2 = a2/a1=q
故,q = 2
將q = 2代入 S奇 = (1-q^2n)/(1-q^2) = 85
(1-2^2n)/(1-4) = 85
求得 2^2n = 2^8
所以,2n = 8
即公比為2,項數(shù)為8