等差数列{an}中,a3+a11=50,则a6+a7+a8=?

问题描述:

等差数列{an}中,a3+a11=50,则a6+a7+a8=?

设等差数列首项为a1,公差为d.
a3+a11=2a1+12d=50
a1+6d=25
a7=25
a6+a7+a8=a7-d+a7+a7+d=3a7=75