①40x=440+y!②30x=310+y

①40x=440+y!②30x=310+y x+x=y+30 X+Y=40,(X*Y)/(X+Y)=7.5 x+x+x+x+y=40 x+y= 13x=y= 多元复合函数微分法z=xyf(x/y,y/x) 求∂z/∂x.∂z/∂x=（∂/∂x)[xyf(x/y,y/x)]=yf(x/y,y/x)+xy(∂/∂x)f(x/y,y/x) 这部是怎么得到的啊?=yf(x/y,y/x)+xy[f①(x/y,y/x)(1/y)+f②(x/y,y/x)(-y/x^2)] 这 x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 第一步是怎么化简成第二步的x²＋y－xy－x①=x(x-y)-(x-y)②=(x-y)(x-1) x+y=450;40%x+30=60%y {x+y=450,60%y-40%x=30 40X+30Y=2200 X>20 Y 化简 x / y(x+y) - y / x(x+y) = 解方程组x+y/6+x-y/10=3,x+y/6+y-x/10=-1x+y/6+x-y/10=3 ①x+y/6+y-x/10=-1 ② y=x(x ①｛ x/2=y/3； 30% ·x -16% ·y=2%② 2x+3y/4+2x-3y/3 =7 ; 2x+3y/3+2x-3y/2=8 2(x+y)-3(X-y)=3①4(X+y)+3(x-y)=15② 30（x+y)=400① 80（y-x)=400② x=? y=?麻烦说一下过程 此方程怎么解 y-(x-y)=3 ① x+(x-y)=15 ② x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空