（1-1/2^2)(1-1/3^2)(1-1/4^2)…(1-1/50^2) =(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)...(1-1/50)(1+1/50)=1/2*3/2*2/3*4/3*3/4*5/4*...*49/50*51/50 =1/2*51/50 =51/100