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f(tx,ty)=(t^n)f(x,y)
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[∂f/∂(tx)]·[d(tx)/dt]+[∂f/∂(ty)]·[d(ty)/dt] = n[t^(n-1)]f(x,y)
[∂f/∂(tx)]·x+[∂f/∂(ty)]·y = n[t^(n-1)]f(x,y)
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{[∂²f/∂(tx)²]·[d(tx)/dt]+[∂²f/∂(tx)∂(ty)]·[d(ty)/dt]}·x
+{[∂²f/∂(ty)∂(tx)]·[d(tx)/dt]+[∂²f/∂(ty)²]·[d(ty)/dt]}·y = n(n-1)[t^(n-2)]f(x,y)
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[∂²f/∂(tx)²]·x² + [∂²f/∂(tx)∂(ty)]·yx +[∂²f/∂(ty)∂(tx)]·xy + [∂²f/∂(ty)²]·y²
=n(n-1)[t^(n-2)]f(x,y)
x²[∂²f/∂(tx)²] + 2xy[∂²f/∂(tx)∂(ty)]+y²[∂²f/∂(ty)²] = n(n-1)[t^(n-2)]f(x,y)
å 为ä¸å¼å¯¹ä»»ä½té½æç«ï¼ä¸å¦¨ä»¤t=1ï¼åï¼
x²(∂²f/∂x²) + 2xy(∂²f/∂x∂y)+y²(∂²f/∂y²) = n(n-1)f(x,y)
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