若等差数列{an}公差d=1/2,a1+a2+a3...+a99=60,求S100
问题描述:
若等差数列{an}公差d=1/2,a1+a2+a3...+a99=60,求S100
答
S99=(a1+a99)*99/2=60
a1+a99=60*2/99
S100=(a1+a100)*100/2=[a1+(a99+d)]*50=(a1+a99)*50+50d
=50*60*2/99+25
=2825/33