等差数列、前n项和为S、a5=5a3、求S4/S5
问题描述:
等差数列、前n项和为S、a5=5a3、求S4/S5
答
a5=a3+2d=5a3
a3=d/2
a4=a3+d=3d/2
a5=5d/2
a2=a3-d=-d/2
a1=a2-d=-3d/2
S4=a1+a2+a3+a4=0
S4/S5=0