列项
1/(1*3)=1/2*(1-1/3)
1/(2*4)=1/2*(1/2-1/4)
1/(3*5)=1/2*(1/3-1/5)
.
1/(98*100)=1/2*(1/98-1/100)
原式
=[1/(1*3)+1/(3*5)+1/(5*7)+...+1/(97*99)]-[1/(2*4)+1/(4*6)+...+1/(98*100)]
=1/2(1-1/3+1/3-1/5+1/5-1/7+...+1/97-1/99)-1/2(1/2-1/4+1/4-1/6+...+1/98-1/100)
=1/2(1-1/99)-1/2(1/2-1/100)
=1/2*98/99-1/2*49/100
=1/2*(98/99-49/100)
=1/2*(49*200-49*99)/9900
=1/2*49*(101)/9900
=4949/19800
=