S3,S4,S2成等差數(shù)列,2S4=S3 +S2,
2*{1/4 *(1-q^4)/(1-q)}= {1/4 *(1-q^3)/(1-q)} + {1/4 *(1-q²)/(1-q)},
2(1-q^4)=(1-q^3) +(1-q²),
2(1+q)(1-q)(1+q²)=(1-q)(1+q+q²)+(1+q)(1-q),-------------------------------------------------------------
2+2q+2q²+2q³=1+q+q²+1+q,
q²(1+2q)=0,
q=-1/2,
an=1/4 *(-1/2)^(n-1)
=(-1)^(n-1) *1/2^(n+1).
標(biāo)-------------------------------------------------------------的化簡(jiǎn)過程兩邊同除了(1-q),
實(shí)際上q=1也可以說是等比,看你老師怎么改了