Sï¼x y zï¼
=ä¸è§å½¢ABCé¢ç§¯-ä¸è§å½¢ARQé¢ç§¯-ä¸è§å½¢BRPé¢ç§¯-ä¸è§å½¢CPQé¢ç§¯
=((æ ¹å·3)/4)a^2-((æ ¹å·3)/4)z(a-y)-((æ ¹å·3)/4)x(a-z)-((æ ¹å·3)/4)y(a-x)
=((æ ¹å·3)/4)(a^2+(xy+yz+zx)-a(x+y+z))
=((æ ¹å·3)/4)(a^2+(xy+yz+zx)-a^2)
=((æ ¹å·3)/4)(xy+yz+zx)
å :(x-y)^2+(y-z)^2+(z-x)^2>=0
æ以:x^2+y^2+z^2>=xy+yz+zx
è:x+y+z=a
(x+y+z)^2=a^2
(x^2+y^2+z^2)+2(xy+yz+zx)=a^2
(xy+yz+zx)+2(xy+yz+zx)<=a^2
xy+yz+zx<=(a^2)/3
æ以:Sï¼x y zï¼=((æ ¹å·3)/4)(xy+yz+zx)<=((æ ¹å·3)/12)a^2
S(X Y Z)çæ大å¼=((æ ¹å·3)/12)a^2
æ¤æ¶x=y=z=a/3
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