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$\displaystyle\int \frac{dx}{e^{x}+e^{-x}}$ is equal to

A$\tan^{-1}(e^{x})+C$
B$\tan^{-1}(e^{-x})+C$
C$\log(e^{x}-e^{-x})+C$
D$\log(e^{x}+e^{-x})+C$
Answer & Solution
Correct answer: A. $\tan^{-1}(e^{x})+C$
1. Multiply numerator and denominator by $e^{x}$: $\dfrac{e^{x}}{e^{2x}+1}$. 2. Put $t=e^{x}$, $dt=e^{x}\,dx$. 3. Integral $=\int \dfrac{dt}{t^2+1}=\tan^{-1}t$. 4. Back-substitute: $\tan^{-1}(e^{x})+C$. _Source: syllabus Mathematics Ch 7 "Integrals", p.350_
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