1/1乘2+1/2乘3+1/3乘4+...+1/49乘50
=1-1/2+1/2+1/3+1/3-1/4+……1/49-1/50
=1-1/50
=49/50
1/2+1/4+1/8+1/16+1/32+1/64==1-1/64=63/64