x+y=450 (1-0.6)x=(1-0.4)y-30这个方程组肿么解呀!

x>=1,x-y y=x-1(x 求导 y=[x/(1+x)]^x y'-(1/x)y=xe^-x z=0.015x*x+y*x*0.05+(1-x-y)*2.5*(x+y) x>0.35.xy>0.70x+y 已知x+y=a,2x-y=-2a,求[（x/y-y/x)/(x+y)-x(1/x-1/y)]/[(x+1)/y]的值 x.y都是自然数,且x(x-u)-y(y-x)+12,求x.y得值快1且x(x-y)-y(y-x)=12 若2/x-1/y=3,求[y/x-y/x-y(x-y/x-x+y)]/x-2y/x的值 y'/y=1/x y(x+y)+(x-y)²-（x+y）（x-y）,其中x=-1/3,y=3 （x-y)(x+y)-(x+y)^2+2y(y-x),其中x=1,y=3. 2y(x+二分之一y)-[(x+y)(x-y)+2y(y+x)],其中|x-1|=2 y=[x-1], x+y=86/x+x/y=1 y=x²-x y=x-1 设x,y满足约束条件x+y>=1,x-y>=-1,2x-y 设x,y满足约束条件x+y>=1,x-y>=-1,2x-y {3(x+y)+3(y+x)=1,3(x+y)+4(y-x)=-1