方法一
=7x²-（4x²-4xy+y²)
=7x²-(2x-y)²
=(√7x+2x-y)(√7x-2x+y)
方法二
=3(x²+4xy/3+4y²/9-7y²/9)
=3[(x+2y/3)²-7y²/9]
=3(x+2y/3+√7y/3)(x+2y/3-√7y/3)
方法一
=7x²-（4x²-4xy+y²)
=7x²-(2x-y)²
=(√7x+2x-y)(√7x-2x+y)
方法二
=3(x²+4xy/3+4y²/9-7y²/9)
=3[(x+2y/3)²-7y²/9]
=3(x+2y/3+√7y/3)(x+2y/3-√7y/3)