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

问题描述:

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