æ¾å¨åæ ç³»ä¸ï¼A(0,0),B(1,0),C(1,1),D(0,1).
设æ£æ¹å½¢å
ä¸ä¸ªç¹M(x,y)ã
MA=(x^2+y^2)^0.5,
MB=((x-1)^2+y^2)^0.5ï¼
MC=((1-x)^2+(1-y)^2)^0.5,
MD=(x^2+(1-y)^2)^0.5;
f(x,y)=MA+MB+MC+MD
=(x^2+y^2)^0.5+((x-1)^2+y^2)^0.5+((1-x)^2+(1-y)^2)^0.5+(x^2+(1-y)^2)^0.5;
xâ(0,1),yâ(0,1).x,yç¸äºç¬ç«ã
æ以å¨å¤çxçæ¶åå¯ä»¥å°yå½æ常æ°ï¼ç¶ååå¤çyã
对ä¸è¿°f(x,y)åå«å¯¹xåyæ±å¯¼
ï¼ä¸ç¥éä½ æ¯å 年级äºï¼ä»¥ååºè¯¥ä¼å¦å°è¿æ¯å¯¹x,yæ±å导ï¼ã
å¦å¯¹x æ±å导记ä½f1'(x)=x/(x^2+y^2)^0.5+(x-1)/((x-1)^2+y^2)^0.5+(x-1)/((1-x)^2+(1-y)^2)^0.5+
x/(x^2+(1-y)^2)^0.5ï¼æ±è§£f1'(x)=0ï¼å¾x=1/2ä¸xâ(0,1/2)ä¸ä¸ºå¢å½æ°ï¼å¨(1/2,1)ä¸å¼åå½æ°ã
ä¹å°±æ¯è¯´x=1/2æ¯å½æ°çæå°å¼ã
对yçåæä¹ä¸æ ·ï¼åå½æ°xï¼yæ¯å æçï¼å¯¹yæ±å¯¼ä¹ååä¸é¢å¯¹xæ±å¯¼åå°xæ¢æyçå¼åä¸æ ·ãæ以(1/2,1/2)æ¯åå½æ°çæå°å¼ç¹ï¼å¸¦å
¥f(x,y)å¾å°f(1/2,1/2)=2*2^0.5.å³2åæ ¹å·2
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