等差数列{an}中,
a3a7=-12 (1)
a3+a7=a4+a6=-4 (2)
(1)(2)联立:a3=2,a7=-6或a3=-6,a7=2
若a3=2,a7=-6
即:
a1+2d=2
a1+6d=-6
解得:a1=6,d=-2
则s20=20*6+20*(20-1)(-2)/2=-260
若a3=-6,a7=2
即:
a1+2d=-6
a1+6d=2
解得:a1=-10,d=2
则s20=20*(-10)+20*(20-1)*2/2=180
s20=-260或180