1/(1*2*3 ) +1/(2*3*4 ) + ...+ 1/（48*49*50）
=1/2[1/(1*2)-1/(2*3)]+1/2[1/(2*3)-1/(3*4)] + ...+ 1/2[1/（48*49）-1/(49*50)]
=1/2[1/(1*2)-1/(49*50)]
=306/1225