Question

已知等差数列公差d>0且a1加a3加a5等于-12,a1a3a5=80求通项an的表达式

Answer 1

a1+a3+a5= -12,即3a3=-12,a3=-4

a3a4a5=18 ,得a4a5=-9/2,即(a3+d)(a3+2d)=-9/2

(-4+d)(-4+2d)=-9/2

16-12d+2d^2=-9/2

2d^2-12d+41/2=0

4d^2-24d+41=0

求出d即可.(d>0)

a1+a5=-12-(-4)=-8.

a1+a1+d=-8

得到a1=...