[tex]9 x^{2} -( x-y)^{2} =9 \\ 9x^{2} -9-(x-y)^{2} \\ (3x-3)(3x+3)-(x-y)(x+y) \\ 9(x-1)(x+1)-(x-y)(x+y) \\ b)4x^2-12xy+9y^2 = (2x-3y)^{2} \\ c)x^4-16 = ( x^{2} -4)( x^{2} +4) \\ d)4x^2-4xy-8y^2= \\ 4x^2+4xy-xy-8y^2= \\ 4x(x+y)-8y(x+y)= \\ =4(x+y)(x-4y) \\ e) 40x^2 + 42x-49 = [/tex]
