(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y)
=(x-y+x-3y)(x-2y)-2(x-2y-y)(x-2y-2y)
=2(x-2y)^2-2(x-2y)^2+4y(x-2y)+2y(x-2y)-4y^2
=4xy-8y^2+2xy-4y^2-4y^2
=6xy-16y^2
=6*4*1.5-16*1.5^2
=0
(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y)
=(x-y+x-3y)(x-2y)-2(x-2y-y)(x-2y-2y)
=2(x-2y)^2-2(x-2y)^2+4y(x-2y)+2y(x-2y)-4y^2
=4xy-8y^2+2xy-4y^2-4y^2
=6xy-16y^2
=6*4*1.5-16*1.5^2
=0