1／（1×2×3）+1／（2×3×4）+1／（4×5×6）、、、、+1／（48×49×50）= 1/2 *[(3-1)/(1*2*3)+(4-2)/(2*3*4).+(50-48)/(48*49*50)]
=1/2 * [1/(1*2)-1/(2*3)+1/(2*3)-1/(3*4).+1/(48*49)-1/(49*50)]
=1/2*(1/2-1/2450)=306/1225