(x-y)^2
=x^2-2xy+y^2
=x^2+2xy+y^2-4xy
=（x+y)^2-4xy
=a^2-4b
x^2+y^2
=x^2+y^2+2xy-2xy
=(x+y)^2-2xy
=a^2-2b
x^2-xy+y^2
=x^2+2xy+y^2-3xy
=(x+y)^2-3xy
=a^2-3b