Question

若数列{an}为等差数列,公差为1／2,且S100=145,则a2+a4+...+a100=

Answer 1

a1+a3+a5+...+a99=s1;a2+a4+...+a100=s2;s1+s2=s100=145,

s1=50*a1+50*(50-1)d,s2=50*a1+50*50*d;

100a1+50*49*0.5+50*50*0.5=145

a1=-23.3代入得s2=50*(25-23.3)=85