等差数列中,a3=5,a7=10,则a5=?a103与7的等差中项为?

Question

等差数列中,a3=5,a7=10 ,则a5=?a10 3与7的等差中项为?

Answer 1

∵an是等差数列；
∴a5=(a3+a7)/2
=(5+10)/2
=15/7
又∵a7-a3=4d
∴d=(a7-a3)/4
=(10-5)/4
=5/4
∴a10=a7+3d
=10+3*(5/4)
=55/4
a3与a7的等差中项是a5