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For the homogeneous function \( f = x^{3} + y^{3} \), the value of \( x f_{x} + y f_{y} \) is

  • A.\( 3f \)
  • B.\( f \)
  • C.\( 2f \)
  • D.\( 6f \)

Answer: A

The function is homogeneous of degree three, so Euler's theorem applies with \( n = 3 \). Checking it directly,

\[ x f_{x} + y f_{y} \;=\; x\left(3x^{2}\right) + y\left(3y^{2}\right) \;=\; 3\left(x^{3} + y^{3}\right) \;=\; 3f. \]

The degree, not the number of variables, sets the multiplier, which is why a quadratic form would return \( 2f \)

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