是..-qan-1把,an+1-an=q(an-an-1),bn=qbn-1,b1=1非0,bn是等比数列bn-1=an-an-1=q^(n-2)类推累加an-a1=1+q+..+q^(n-2),an=1+(1-q^(n-1))/(1-q)
