a3^2=a2*a7
(a1+2d)^2 = (a1+d) (a1+6d)
d=-3/2 a1
q=a3 / a2=(a1+2d)/(a1+d)=(a1-3a1)/(a1- 3/2 a1)=4
a2=a1+d,a3=a1+2d,a7=a1+6d
a2，a3,a7为等比数列
a3^2=a1*a7
(a1+2d,)^2=(a1+d)(a1+6d
5a1d=-2d^2
5a1=-2d a1=-2d/5 a2=3d/5 q=-2/3
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