3y=x+2y
错了吧
是不是3y=x+2z?
x-3y+2z=0
x²+9y²+4z²-6xy-12yz+4xy
=(x²-6xy+9y²)-12yz+4xy+4z²
=(x-3y)²-4z(x-3y)+4z²
=(x-3y+2z)²
=0²=0