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The tangent plane to \( x^{2} + y^{2} + z^{2} = 9 \) at \( (1, 2, 2) \) has equation

  • A.\( x + 2y + 2z = 9 \)
  • B.\( x + 2y + 2z = 3 \)
  • C.\( 2x + 4y + 4z = 9 \)
  • D.\( x + y + z = 5 \)

Answer: A

The gradient \( 2\mathbf{i} + 4\mathbf{j} + 4\mathbf{k} \) is normal there, and dividing by two gives the simpler normal \( (1, 2, 2) \). The plane through the point with that normal is

\[ 1(x-1) + 2(y-2) + 2(z-2) \;=\; 0 \quad \Longrightarrow \quad x + 2y + 2z \;=\; 9. \]

Substituting the point confirms \( 1 + 4 + 4 = 9 \)

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