1/2+1/8+1/24+1/48+1/80

题目

1/2+1/8+1/24+1/48+1/80
要简便哦

答案

1/2+1/8+1/24+1/48+1/80
=1/1*2+1/2*4+1/4*6+1/6*8+1/8*10
=1/2*(1-1/2+1/2-1/4+……+1/8-1/10)
=1/2*(1-1/10)
=1/2*9/10
=9/20