a4=7,a1,a2 ,a5成等比数列
(7-2d)²=(7-3d)(7+d) d=2
an=2n-1
a2^n=2^n*2-1
Tn=(4-2*2*2^n)/(1-2)-n=4*2^n-4-n7－２d　不是成了a2了吗？　　怎么是比例中项呢？a1,a2 ,a5成等比数列???????恩a1*q=a2a1*q*q=a5a1*a5=a1*a1*q*q=a2*a2