1*1/2*2/3*3/4*4/5*5/6......*47/48*48/49*49/50=1*1/50=1/50
1·(1-1/2)(1-1/3)(1-1/4)(1-1/5)(1-1/6)(1-1/7)(1-1/8)........(1-1/48)(1-1/49)(1-1/50)=1/50
