用平方差
原式=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/2009)(1+1/2009)
=(1/2)(3/2)(2/3)(4/3)……(2008/2009)(2010/2009)
=(1/2)(2010/2009)
=1005/2009
用平方差
原式=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/2009)(1+1/2009)
=(1/2)(3/2)(2/3)(4/3)……(2008/2009)(2010/2009)
=(1/2)(2010/2009)
=1005/2009