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The integral \( \displaystyle\int \frac{dx}{a^{2} + x^{2}} \) equals

  • A.\( a\tan^{-1}\dfrac{x}{a} + C \)
  • B.\( \dfrac{1}{a}\tan^{-1}\dfrac{x}{a} + C \)
  • C.\( \dfrac{1}{a}\sin^{-1}\dfrac{x}{a} + C \)
  • D.\( \dfrac{1}{2a}\ln\dfrac{a+x}{a-x} + C \)

Answer: B

Substituting \( x = a\tan\theta \) gives \( dx = a\sec^{2}\theta\,d\theta \) and turns the denominator into \( a^{2}\sec^{2}\theta \), leaving

\[ \int \frac{a\sec^{2}\theta\,d\theta}{a^{2}\sec^{2}\theta} \;=\; \frac{\theta}{a} \;=\; \frac{1}{a}\tan^{-1}\frac{x}{a} + C. \]

The inverse sine belongs to \( 1/\sqrt{a^{2}-x^{2}} \), and the logarithm to \( 1/(a^{2}-x^{2}) \)

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