在等差数列{An}中,若 S12=8*S4,则A1/d=?

问题描述:

在等差数列{An}中,若 S12=8*S4,则A1/d=?

S12=8S4
(a1+a12)*12/2=8*(a1+a4)*4/2
3a1+3a12=8a1+8a4
3(a1+11d)=5a1+8(a1+3d)
3a1+33d=5a1+8a1+24d
10a1=9d
a1/d=9/10