Question

等差数列的前n项和Sn,a8=2a3,则,S15/S5的值?

Answer 1

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