x + y =498；0.7x + 0.9y =386.x,y是多少

x + y =498；0.7x + 0.9y =386.x,y是多少 x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 化简 x / y(x+y) - y / x(x+y) = y=x(x x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空 x+x+x+x+y+y+y=56 已知实数x,y满足方程x*x+y*y-4x+1=0.求y-x的最大值 设x.y满足条件y>=0.X-Y+1>=0.x+y-3 在约束条件x>=0.y>=0.y+x 在约束条件x>=0.y>=0.y+x 在约束条件x>=0.y>=0.y+x 推倒公式:(x+y)(x*x-3x+y*y)=x*x*x+y*y*y怎么做. ｛x+y=100 0.7x+0.8y=74 ｛x-y=-3.7x+5y=0. x+x+x+y+y=6.4 y+y+y+x+x=7.1 怎么算? 二元函数f(x,y)=x+y/x-y,求f(y/x,x/y) x-y/x+y=(x-y)²/x²-y²（x≠y) （x+y-5)*（x+y-5)+（x-y+3)=0 x*x-y*y