（1-1/2^2）(1-1/3^2)(1-1/4^2).......(1-1/2004^2)=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)....(1-1/2004)(1+1/2004)=1/2*3/2*2/3*4/3*3/4*5/4*.....*2003/2004*2005/2004=1/2*2005/2004=2005/4008
