(1-1/2^2)(1-1/3^2)...(1-1/1999^2)(1-1/2000^2) =(1-1/2)(1+1/2)*(1-1/3)(1+1/3)*...*(1-1/1999)(1+1/1999)*(1-1/2000)(1+1/2000) =(1/2*3/*2)*(2/3*4/3)*...*(1998/1999*2000/1999)*(1999/2000*2001/2000) =1/2*(3/2*2/3)*(4/3*3/4)*...*(2000/1999*1999/2000)*2001/2000 =1/2*2001/2000 =2001/4000