已知(a1+2d)(a1+6d)=-12与(a1+3d)+(a1+5d)=-4为方程组,求a1与d?

问题描述:

已知(a1+2d)(a1+6d)=-12与(a1+3d)+(a1+5d)=-4为方程组,求a1与d?

(a1+3d)+(a1+5d)=-4=》2a1+8d=-4===>a1+4d=-2
所以 a1+2d=-2-2d a1+6d=-2+2d
(a1+2d)(a1+6d)=(-2-2d )(-2+2d )=4-4d2=-12==>d=+-2
当d=2时候 a1=-10 当d=-2时 a1=6