(x+1)/20=(y+1)/21=(x+y)/17
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(x+1)/20=(y+1)/21=(x+y)/17(x+1)/20=(y+1)/21=(x+y)/17(x+1)/20=(y+1)/21=(x+y)/17(x+1)/20=(y+1)/2121(x+
(x+1)/20=(y+1)/21=(x+y)/17
(x+1)/20=(y+1)/21=(x+y)/17
(x+1)/20=(y+1)/21=(x+y)/17
(x+1)/20=(y+1)/21
21(x+1)=20(y+1)
21x+21=20y+20
21x-20y=-1
(y+1)/21=(x+y)/17
17(y+1)=21(x+y)
17y+17=21x+21y
21x+4y=17
相减
24y=18
y=3/4
x=(17-4y)/21=2/3
{6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1
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=20x X（1-x)的最大值?
1=x+y 2=20x+30y
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
解方程组6(x-y)-7(x+y)=21 2(x-y)-5(x+y)=-1
1/(x+y)-1/(x-y)=-3/20,2/(x+y)+3/(y-x)=-(1/5)急啊!
y=[x-1],