已知等差数列{An}的公差为1/2,S1oo=145,则a2+a4+.+a1oo的值为?
问题描述:
已知等差数列{An}的公差为1/2,S1oo=145,则a2+a4+.+a1oo的值为?
答
a1=a2-d,a3=a4-d…a99=a100-d;
s100=a1+a2…a100=a2-d+a2+a4-d+a4+…+a100-d+a100
=2(a2+a4+…+a100)-50d
=145
a2+a4+…+a100=(145+50÷2)÷2=85