ПОМОГИТЕ(2х+у)^3+(x-2y)^3=?(2mn-1)^3+1=?a^2+4b^2-9c^2-4ab=?x^3+x^2-xy^2-y^2=?

1) 
a + b = (a+b)(a -ab+b)
a=2x+y
b=x-2y
(2x+y) + (x-2y)=(2x+y+x-2y)((2x+y) -(2x+y)(x-2y)+(x-2y))=
=(3x-y)(4x+4xy+y -(2x+xy-4xy-2y)+x -4xy+4y)=
=(3x-y)(4x +4xy+y -2x +3xy +2y +x -4xy+4y)=
=(3x-y)(5x -2x +7y +3xy)=15x-5xy-6x+2xy+21xy-7y+9xy-3xy=
=15x -6x -7y +2xy+4xy+18xy

2)
(2mn-1) +1 =(2mn-1+1)((2mn-1) -(2mn-1)*1+1)=
=2mn(4mn-4mn+1-2mn+1+1)=2mn(4mn-6mn+3)=8mn-12mn+6mn

3)
a+4b -9c -4ab=(a -4ab+4b)-9c =(a-2b) -9c =(a-2b-3c)(a-2b+3c)

4)
x +x -xy -y =(x +x)-(xy +y)=x(x+1)-y(x+1)=(x+1)(x -y)=
=(x+1)(x-y)(x+y)