m²-mn+mx-nx=m(m-n)+x(m-n)=(m+x)(m-n)
x²-y²-3x-3y=(x+y)(x-y)-3(x+y)=(x+y)(x-y-3)
x=√3+1,y=√3-1,求：x²-y²= (√3+1)²-(√3-1)²=3+2√3+1-(3-2√3+1)=4√3
（x²-y²-6x-6y)÷（x+y)=((x+y)(x-y)-6(x+y))÷（x+y)=(x+y)(x-y-6)÷（x+y)=x-y-6
[題目告訴你（x+y)不為0]
4x²=(x-1)² 4x²=x²-2x+1 3x²+2x-1=0 （3x-1)(x+1)=0 x=1/3 or x=-1
4a²+1=4a 4a²-4a+1= 0 (2a-1)²=0 a1=a2=1/2