4乘8分之1应该是4乘6分之1
原式=(1/2)(1/2-1/4)+(1/2)(1/4-1/6)+……+(1/2)(1/18-1/20)
=(1/2)(1/2-1/4+1/4-1/6+……+1/18-1/20)
=(1/2)(1/2-1/20)
=9/40