等差数列的前n项和Sn,a8=2a3,则,S15/S5的值?

等差数列的前n项和Sn,a8=2a3,则,S15/S5的值?
等差数列的前n项和Sn,a8=2a3,则,S15/S5的值?

等差数列的前n项和Sn,a8=2a3,则,S15/S5的值?
a8=2a3
a3+5d=2a3
a3=5d
a1+2d=5d
a1=3d
s15/s5
=[(a1+a15)*15/2]/[(a1+a5)*5/2]
=3(a1+a15)/(a1+a5)
=3(2a1+14d)/(2a1+4d)
=3(2*3d+14d)/(2*3d+4d)
=3*20d/(10d)
=6
s15/s5
=[(a1+a15)*15/2]/[(a1+a5)*5/2]
=3(a1+a15)/(a1+a5)
=3*2a8/(2a3)
=3*a8/a3
=3*2a3/a3
=3*2
=6
=6