已知数列{an}是等差数列,若a1+a3+a8=15,则a4等于?

已知数列{an}是等差数列,若a1+a3+a8=15,则a4等于?
已知数列{an}是等差数列,若a1+a3+a8=15,则a4等于?

已知数列{an}是等差数列,若a1+a3+a8=15,则a4等于?
a1+a3+a8
=a1+(a1+2d)+(a1+7d)
=3a1+9d=15
所以a1+3d=5
所以a4=a1+3d=5

a3=a1+2d
a8=a1+7d
a4=a1+d
因为a1+a3+a8=15
即a1+a1+2d+a1+7d=15
所以3a1+9d=15
所以a1+3d=5
则a4=5

a4=5.

a1+a3+a8=a4+a4+a4=15
a4=5