(1-1/4)(1-1/9)(1-1/16).......(1-1/10000)=(1-1/2^2)(1-1/3^2)......(1-1/20^2)=(1-1/2)(1+1/2)(1-1/3)(1+1/3).....(1-1/20)(1+1/20)=1/2*3/2*2/3*4/3.....19/20*21/20=21/2
