=[(x+2)/x(x-2)-(x-1)/(x-2)^2]/[(x-4)/x]=[(x+2)(x-2)/x(x-2)^2-(x-1)x/(x-2)^2x]/[(x-4)/x]=[(x^2-4)/x(x-2)^2-(x^2-x)/(x-2)^2x][x/(x-4)]=[(x^2-4)-(x^2-x)]/(x-2)^2x乘以[x/(x-4)]=1/(x-2)^2