设公比为q,项数为2n
a1(q^2n-1)/(q-1) = 4 a1q(q^2n -1) /(q^2 -1)
4q = q+1
q = 1/3
前3项之积等于64,a1^3 * q^3 = 64
a1q = 4
a1 = 12
这个数列 an = 4*1/3^(n-1)