狠狠狠狠狠狠狠狠难. （1）设x，y，z为正实数.求证x^2/(x^2+y^2+xy)+y^2/(y^2+z^2+yz)+z^2/(z^2+x^2+zx)>；=1证明 去分母得：x^2*(y^2+z^2+yz)*(z^2+x^2+zx)+y^2*(z^2+x^2+zx)*(x^2+y^2+xy)+z^2*(x^2+y^2+xy)(y^2+z^2+yz)>；=(y^2+z^2+yz)*(z^2+.
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Where are these countries from？Asia(亚洲)_Europe(欧洲)_North America(北美洲)_South America(南美洲)_
