Derivative of Arcsecant Function

Theorem
Let $x \in \R$ be a real number such that $\size x > 1$.

Let $\arcsec x$ be the arcsecant of $x$.

Then:
 * $\dfrac {\map \d {\arcsec x} } {\d x} = \dfrac 1 {\size x \sqrt {x^2 - 1} } = \begin {cases} \dfrac {+1} {x \sqrt {x^2 - 1} } & : 0 < \arcsec x < \dfrac \pi 2 \ (\text {that is: $x > 1$}) \\

\dfrac {-1} {x \sqrt {x^2 - 1} } & : \dfrac \pi 2 < \arcsec x < \pi \ (\text {that is: $x < -1$}) \\ \end{cases}$

Proof
Let $y = \arcsec x$ where $\size x > 1$.

Then:

Since $\dfrac {\d y} {\d x} = \dfrac 1 {\sec y \tan y}$, the sign of $\dfrac {\d y} {\d x}$ is the same as the sign of $\sec y \tan y$.

Writing $\sec y \tan y$ as $\dfrac {\sin y} {\cos^2 y}$, it is evident that the sign of $\dfrac {\d y} {\d x}$ is the same as the sign of $\sin y$.

From Sine and Cosine are Periodic on Reals, $\sin y$ is never negative on its domain ($y \in \closedint 0 \pi \land y \ne \pi/2$).

However, by definition of the arcsecant of $x$:
 * $0 < \arcsec x < \dfrac \pi 2 \implies x > 1$
 * $\dfrac \pi 2 < \arcsec x < \pi \implies x < -1$

Thus:


 * $\dfrac {\map \d {\arcsec x} } {\d x} = \dfrac 1 {\size x \sqrt {x^2 - 1} } = \begin{cases} \dfrac {+1} {x \sqrt {x^2 - 1} } & : 0 < \arcsec x < \dfrac \pi 2 \ (\text {that is: $x > 1$}) \\

\dfrac {-1} {x \sqrt {x^2 - 1} } & : \dfrac \pi 2 < \arcsec x < \pi \ (\text {that is: $x < -1$}) \\ \end{cases}$

Hence the result.

Also see

 * Derivative of Arcsine Function
 * Derivative of Arccosine Function
 * Derivative of Arctangent Function
 * Derivative of Arccotangent Function
 * Derivative of Arccosecant Function