Question

等差数列中,a1=-3,11a5=5a8,则sn

Answer 1

11a5=5a8

11*(a1+4d)=5*(a1+7d)

11a1+44d=5a1+35d

6a1=-9d

2a1=-3d

2*(-3)=-3d

d=2

an=a1+(n-1)d

=-3+2(n-1)

=2n-5

sn=(a1+an)*n/2

=(-3+2n-5)*n/2

=n(n-4)