先化简再求值(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y) x=4 y=5

(X-y)(x-2y)+(X-2Y)(X-3Y)-2(X-3Y)(X-4Y)
=(x-2y)(x-y+x-3y)-2(x-3y)(x-4y)
=(x-2y)(2x-4y)-2(x-3y)(x-4y)
=2x²-4xy-4xy+8y²-2(x²-4xy-3xy+12y²)
=2x²-4xy-4xy+8y²-2x²+8xy+6xy-24y²
=6xy-16y²
=6x4x5-16x5²
=120-400
=-280确定对？嗯哪，学习进步~thank you满意请采纳哦~