已知x=m²-2mn+n²,y=m²-2mn-n².当m=1,n=2时,求x-[y-2x-(x-y)]的值

已知x=m²-2mn+n²,y=m²-2mn-n².当m=1,n=2时,求x-[y-2x-(x-y)]的值
已知x=m²-2mn+n²,y=m²-2mn-n².当m=1,n=2时,求x-[y-2x-(x-y)]的值

已知x=m²-2mn+n²,y=m²-2mn-n².当m=1,n=2时,求x-[y-2x-(x-y)]的值
x-[y-2x-(x-y)]
=x-y+2x+x-y
=4x-2y
=4(m²-2mn+n²)-2(m²-2mn-n²)
=4m²-8mn+4n²-2m²+4mn+2n²
=2m²-4mn+6n²
=2-8+24
=18

x-[y-2x-(x-y)]
=x-y+2x+x-y
=4x-2y
=4(m²-2mn+n²)-2(m²-2mn-n²)
=2m²-4mn+6n²
=2×1²-4×1×2+6×2²
=2-8+24
=18