Question

等差数列｛an｝中,若a2+a4+a15的值为常数,则下各数中也为常数的是：

Answer 1

a2+a4+a15

=a2+(a2+2d)+(a2+13d)

=3·a2+15d

=3·(a2+5d)

=3·a7

则a7是确定的常数.

2·a7

=2·(a1+6d)

=(a1+6d)+(a1+6d)

=a1+(a1+12d)

=a1+a13

即a1+a13是确定的常数.

则:

S13=13/2·(a1+a13)

是确定的常数.