數學問題---抽因子

1.(2a+b)^2-(a-b)^2
=(2a+b-a+b)(2a+b+a-b)
=(a+2b)(3a)
2.(2x-y)^2-(x+2y)^2
=(2x-y-x-2y)(2x-y+x+2y)
=(x-3y)(3x+y)
3.(x-y)^2-8(x-y)+16
=(x-y-4)(x-y-4)
=(x-y-4)^2
4.4(2x+y)^2+4(2x+y)+1
=[2(2x+y)+1][2(2x+y)+1]
=(8x+8x+1)^2
5.4x^2-y^2-8x-4y
這題好似打錯問題...
6.25x^2-(9y^2-6y+1)
=25x^2-(3y-1)^2
=(25x+3y-1)(25x-3y+1)
7.4x^2-(4y^2-4yz+z^2)
=4x^2-(4y-z)^2
=(4x+4y-z)(4x-4y+z)

為自由人加一條
5) 4x^2-y^2-8x-4y
4x^2-y^2=(2x)^2-y^2=(2x+y)(2x-y)
-8x-4y=-4(2x+y)
結合一齊:4x^2-y^2-8x-4y=(2x+y)(2x-y)-4(2x+y)
=(2x+y)(2x-y-4)