Question

已知等差数列｛an｝满足a2+a4=4,a3+a5=10,则它的前10项的和S10等于多少.

Answer 1

设公差为d

a2+a4=4

a1+d+a1+3d=4

2a1+4d=4.1

a3+a5=10

a1+2d+a1+4d=10

2a1+6d=10.2

d=3 a1=-4

S10=a1n+n(n-1)d/2=(-4)n+n(n-1)*3/2 =-40+10*9*3/2=-40+135=95