1/(1*2*3*4)=1/3(1/1*2*3-1/2*3*4)1/(2*3*4*5)=1/3(1/2*3*4-1/3*4*5)......1/(1*2*3*4)+1/(2*3*4*5)+1/(3*4*5*6).+1/(17*18*19*20)=1/3(1/1*2*3-1/18*19*20)=1/3*(3*19*20-1)/(18*19*20)=1139/20520