1：900^2-1800*892+892^2
=900^2-2*900*892+892^2
=(900-892)^2
=8^2
=64;
2：（1-1/2^2）*（1-1/3^2）……（1-1/2005^2）(1-1/2006^2)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3).(1-1/2006)(1+1/2006)
=(1/2)(3/2)(2/3)(4/3).(2005/2006)(2007/2006)
=(1/2)(2007/2006)
=2007/4012.