設(shè)這個等差數(shù)列的公差為d
那么 a2=a1+d a4=a1+3d a5=a1+4d
a1+a5=a1+a1+4d=2a1+4d=2
a2a4=(a1+d)(a1+3d)=-3
2a1+4d=a1+2d=1 a1=1-2d帶入 (a1+d)(a1+3d)=-3
得
(1-d)(1+d)=-3
1-d^2=-3
d^2=4
d=2或 d=-2
當(dāng)d=2時 a1=-3
an=-3+2(n-1)=2n-5
當(dāng)d=-2時 a1=5
an=5-2(n-1)=7-2n