2x^2+2y^2
=2(x^2+y^2)
=2[(x+y)^2-2xy]
=2(7^2-2×2)
=2×45
=90
(x-y)^2=(x+y)^2-4xy
=7^2-4×2
=49-8
=41
已知x+y=7，xy=2，求下列各式的值：
2x^2+2y^2
=2(x^2+y^2)
=2[(x+y)^2-2xy]
=2(7^2-2×2)
=2×45
=90
(x-y)^2=(x+y)^2-4xy
=7^2-4×2
=49-8
=41