设第一项为a1,公差为d d>0
则a2=a1+d a3=a1+2d a4=a1+3d
a2+a3=2a1+3d=2
a1*a4=a1*(a1+3d)=-8
a1+3d=2-a1
a1*(a1+3d)=a1(2-a1)=-8
-a1^2+2a1+8=0
a1^2-2a1-8=0
(a1-4)(a1+2)=0
a1=4 或a1=-2
由a1*a4=-8还有更简便的算法没呀很简单了