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None of these four is a valid substitution for \( \int \sqrt{a^{2} - x^{2}}\,dx \) except

  • A.\( x = a\cosh u \)
  • B.\( x = a\sin\theta \)
  • C.\( x = a\tan\theta \)
  • D.\( x = a\sec\theta \)

Answer: B

With \( x = a\sin\theta \) the radical becomes \( a\cos\theta \), removing the root cleanly. The tangent substitution suits \( a^{2} + x^{2} \), and the secant and hyperbolic cosine suit \( x^{2} - a^{2} \); each of those three leaves the wrong sign under the root here and produces an imaginary result

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