1/1*2+1/2*3+1/3*4+.+1/99*100
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/99-1/100)
=11-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
=1-1/100
=99/100