`a,4x - 4y + x^2 (y - x)`
`=4(x-y)-x^2(x-y)`
`=(x-y)(4-x^2)`
`=(x-y)(2-x)(2+x).`
`b,25(x+y)^2 - 16(x-y)^2`
`=[5(x+y)]^2 - [4(x-y)]^2`
`=[5(x+y)-4(x-y)][5(x+y)+4(x-y)]`
`=(5x+5y-4x+4y)(5x+5y+4x-4y)`
`=(x+9y)(9x+y).`
`c)2x-8x-42`
`=-6x-42`
`=-6(x+7).`
`d)x^4+2013x^2+2011x^2 + 2012`
`=x^4+4024x^2+ 2012`
`=x^4+2.2012x^2+ 2012^2-4046132`
`=(x^2+2012)^2-(\sqrt{4046132})^2`
`=(x^2+2012-\sqrt{4046132})(x^2+2012+\sqrt{4046132}).`
`g)x^4-y^4`
`=(x^2)^2-(y^2)^2`
`=(x^2+y^2)(x^2-y^2)`
`=(x^2+y^2)(x-1)(x+1).`
