设n-2006=a 2008-n=b
a^2+b^2=1 ==> (a+b)^2=a^2+b^2+2ab=1+2ab =(n-2006+2008-n)^2=2^2=4
2ab=3 ab=3/2
2008减n 乘以n减2006 =3/2