Question

一个有15项的等差数列,第8项是50,这个数列的各项的和是多少?

Answer 1

a1+a15=2a8

a1+a15=2a8

a2+a14=2a8

a3+a13=2a8

a4+a12=2a8

a5+a11=2a8

a6+a10=2a8

a1+a2+……+a15

=(a1+a15)+(a2+a14)+(a3+a13)+(a4+a12)+(a5+a11)+(a6+a10)+(a7+a9)+a8

=15a8

=750