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The stationary point of \( f(x,y) = x^{2} + y^{2} - 4x - 6y + 20 \) lies at

  • A.\( (4, 6) \)
  • B.\( (1, 1.5) \)
  • C.\( (2, 3) \)
  • D.\( (-2, -3) \)

Answer: C

Set both first derivatives to zero:

\[ f_{x} \;=\; 2x - 4 \;=\; 0, \qquad f_{y} \;=\; 2y - 6 \;=\; 0, \]

giving \( x = 2 \) and \( y = 3 \). Since \( f_{xx} = f_{yy} = 2 \) and \( f_{xy} = 0 \), the discriminant is \( D = 4 > 0 \) with \( f_{xx} > 0 \), so the point is a minimum, of value seven

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