计算已知等差数列{an}的公差为1/2且S100=145则a1+
已知等差数列{an}的公差为1/2且S100=5则a1+a3+a5+.....+a99=?
Sn=na1+n(n-1)d/2 145=100*a1+100*99/4 a1=-23.3 a3-a1=2d=1 a1+a3+a5+.....+a99 =50*(-23.3)+50*49/2 =60