X-Y=500 (1+15%)X-(1-10%)Y=950

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 x-3y=2x+y-15=1 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 10/x+y+3/x-y=-5 15/x+y-2/x-y=-1（用换元法解此方程组） 解分式方程：10/(x+y)+3/(x-y)=-5,15/(x+y)-2/(x-y)=-1 3/(X-Y)+10/(X+Y)=-5 2/(X-Y)-15/(X+Y)=1 解分式方程组 x+y/10+x/15=1x+y/12+y/15=1 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 x-2y=15,xy=-25,求x*x+4y*y-1