1.=(x+2y)²+2xz+4yz+z²
=x²+4xy+4y²+2xz+4yz+z²
2.=(2x-3y)²+16z²-2(2x-3y)*4z
=4x²+9y²-12xy-16xz+24yz
3.同理=y²+4x²+12x-4xy-6y+9