20x+50y+z=16,x+y+z=100 请高手们给出尽量多的答案,小弟急用!

(x+y-z)(x-y+z)= x+y+z=14,x'+y'+z'=15,(x-x')+(y+y')+z*z'=16,已知xyz为自然数,求x,y,z的 x y z x+y--- = --- = ---- ----y+Z z+x x+y ,求 z 的值 .求 x+y---- X+Y+Z=? x+y+z=1 求xyz/(x+y)(y+z)(z+x)的最大值 若xy-z不等于0,且(y+x)/x=(z+x)/y=(y+x)/z,求[(y+z)(z+x)(x+y)]/xyz的值? x+2y+3z=20,x+3y+5z=31,求x+y+z的值 z=f(x,y)是方程e^(-xy)-2z+e^z给出的函数,求全微分dz 已知x、y、z满足方程组：x+y-z=6；y+z-x=2；z+x-y=0 求x、y、z的值 f(x,y,z,w)=x*(x+y)*(x+y+z)*(x+y+z+w) 若xyz不等于0,且满足(y+z)/x=(x+z)/y=(x+y)/z,求（y+z)（x+z)（x+y)/xyz的值 若xyz不等于0,且(y+z)/x=(z+x)/y=(x+y)/z,求(y+z)(z+x)(x+y)/xyz的值?