9x的二次方-12xy+4y的二次方-4x的二次方+12xy-9y的二次方
=(9x²-4x²)+(12xy-12xy)+(4y²-9y²)
=5x²-5y²
=5x(1/2)²-5x(1/2)²
=5/4-5/4
=0