a1(1-q5)/1-q + a1(1-q6)/1-q = 2a1(1-q4)/1-q
(a1-a1q5+ a1-a1q6)/1-q = (2a1-2a1q4)/1-q
2a1-a1q5-a1q6 = 2a1-2a1q4
2a1q4 =a1q5+ a1q6
a1q4(2) = a1q4(q+q2）
q2+q-2 =0
q=1 ,q=-2
q4,q5,q6,q2,是q的几次方