x²-3xy-7y²=x²-3xy+9y²/4--9y²/4-7y²=(x-3y/2)²-37y²/4=(x-3y/2+v37y/2)(x-3y/2-v37y/2)用配方法分解因式
x²-3xy-7y²=x²-3xy+9y²/4-9y²/4-7y²=(x-3y/2)²-37y²/4=(x-3y/2+√37*y/2)(x-3y/2-√37*y/2)
