Question

若等差数列an中,公差d>0,且a1+a3+a5= -12,a3a4a5=18 求通项公式

Answer 1

a1+a3+a5= -12,即3a3=-12,a3=-4a3a4a5=18 ,得a4a5=-9/2,即(a3+d)(a3+2d)=-9/2(-4+d)(-4+2d)=-9/216-12d+2d^2=-9/22d^2-12d+41/2=04d^2-24d+41=0求出d即可.(d>0)a1+a5=-12-(-4)=-8.a1+a1+d=-8得到a1=...