1又1×2分之1+2又2×3分之1+3又3×4分之1+……+49又49×50分之1
=(1+2+3+...+49)+[1/(1*2)+1/(2*3)+...+1/(49*50)]
=(1+49)*49/2+(1-1/2+1/2-1/3+...+1/49-1/50)
=1225+(1-1/50)
=1225+49/50
=1225又49/50