(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)=

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(x-y)(x-2y)+(x+2y)(x-3y)-2(x-3y)(x-4y)=(x-y)(x-2y)+(x+2y)(x-3y)-2(x-3y)(x-4y)=(x-y)(x-2y)+(x+2y)(x-3

(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)=
(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)=

(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)=
(x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)
=(x-y) (x-2y)+(x-3y)[(x+2y) -2(x-4y)]
=(x-y)(x-2y)+(x-3y)[10y-x]
=x^2-xy-2xy+2y^2+10xy-3y^2-x^2+3xy
=-y^2+10xy
=y(10x-y)