4x-3y=6z 28x-21y=42z2x+4y=14z6x+12y=42z28x-21y=6x+12y28x-6x=12y+21y22x=33y2x=3yx/y=3/22xy/(x^2+y^2)分子分母同时除以y^2=(2xy/y^2)/(x^2/y^2+y^2/y^2)=(2x/y)/(x^2/y^2+1)=(2*3/2)/[(3/2)^2+1]=3/(9/4+1)=3/(1...