等差数列{an}的公差d≠0,比较a4a9与a6a7的大小

问题描述:

等差数列{an}的公差d≠0,比较a4a9与a6a7的大小

a4a9=(a1+3d)(a1+8d)=a1^2+11a1d+24d^2
a6a7=(a1+5d)(a1+6d)=a1^2+11a1d+30d^2
a4a9-a6a7=a1^2+11a1d+24d^2-(a1^2+11a1d+30d^2)
=-6d^2a4a9