B.3 Inverse Trigonometric Functions

In order to construct inverse trigonometric functions we first have to restrict their domains so as to make them one-to-one (or injective). We do this as shown below

Since these functions are inverses of each other we have

\begin{align*} \arcsin(\sin \theta) &= \theta & -\frac{\pi}{2} \leq \theta \leq \frac{\pi}{2}\\ \arccos(\cos \theta) &= \theta & 0 \leq \theta \leq \pi\\ \arctan(\tan \theta) &= \theta & -\frac{\pi}{2} \leq \theta \leq \frac{\pi}{2} \end{align*}

and also

\begin{align*} \sin(\arcsin x) &= x & -1 \leq x \leq 1\\ \cos(\arccos x) &= x & -1 \leq x \leq 1\\ \tan(\arctan x) &= x & \text{any real $x$} \end{align*}

We can read other combinations of trig functions and their inverses, like, for example, \(\cos(\arcsin x)\text{,}\) off of triangles like

We have chosen the hypotenuse and opposite sides of the triangle to be of length 1 and \(x\text{,}\) respectively, so that \(\sin(\theta)=x\text{.}\) That is, \(\theta = \arcsin x\text{.}\) We can then read off of the triangle that

\begin{align*} \cos(\arcsin x) &= \cos(\theta) = \sqrt{1-x^2} \end{align*}

We can reach the same conclusion using trig identities, as follows.

• Write \(\arcsin x=\theta\text{.}\) We know that \(\sin(\theta)=x\) and we wish to compute \(\cos(\theta)\text{.}\) So we just need to express \(\cos(\theta)\) in terms of \(\sin(\theta)\text{.}\)
• To do this we make use of one of the Pythagorean identities

  \begin{align*} \sin^2\theta + \cos^2\theta &=1\\ \cos\theta &= \pm \sqrt{1-\sin^2\theta} \end{align*}

• Thus

  \begin{gather*} \cos(\arcsin x) = \cos\theta = \pm\sqrt{1-\sin^2\theta} \end{gather*}

• To determine which branch we should use we need to consider the domain and range of \(\arcsin x\text{:}\)

  \begin{align*} \text{Domain: } -1 \leq x \leq 1 && \text{Range: } -\frac{\pi}{2} \leq \arcsin x \leq \frac{\pi}{2} \end{align*}

  Thus we are applying cosine to an angle that always lies between \(-\frac{\pi}{2}\) and \(\frac{\pi}{2}\text{.}\) Cosine is non-negative on this range. Hence we should take the positive branch and

  \begin{align*} \cos(\arcsin x) &= \sqrt{1-\sin^2\theta}= \sqrt{1-\sin^2(\arcsin x)}\\ &= \sqrt{1-x^2} \end{align*}

In a very similar way we can simplify \(\tan(\arccos x)\text{.}\)

• Write \(\arccos x=\theta\text{,}\) and then

  \begin{align*} \tan( \arccos x) &= \tan \theta = \frac{\sin\theta}{\cos \theta} \end{align*}

• Now the denominator is easy since \(\cos \theta = \cos \arccos x = x\text{.}\)
• The numerator is almost the same as the previous computation.

  \begin{align*} \sin\theta &= \pm \sqrt{1-\cos^2\theta}\\ &= \pm \sqrt{1-x^2} \end{align*}

• To determine which branch we again consider domains and and ranges:

  \begin{align*} \text{Domain: } -1 \leq x \leq 1 && \text{Range: } 0 \leq \arccos x \leq \pi \end{align*}

  Thus we are applying sine to an angle that always lies between \(0\) and \(\pi\text{.}\) Sine is non-negative on this range and so we take the positive branch.
• Putting everything back together gives

  \begin{align*} \tan(\arccos x) &= \frac{\sqrt{1-x^2}}{x} \end{align*}

Completing the 9 possibilities gives:

\begin{align*} \sin( \arcsin x ) &= x & \sin( \arccos x ) &= \sqrt{1-x^2} & \sin( \arctan x ) &= \frac{x}{\sqrt{1+x^2}}\\ \cos( \arcsin x ) &= \sqrt{1-x^2} & \cos( \arccos x ) &= x & \cos( \arctan x ) &= \frac{1}{\sqrt{1+x^2}}\\ \tan( \arcsin x ) &= \frac{x}{\sqrt{1-x^2}} & \tan( \arccos x ) &= \frac{\sqrt{1-x^2}}{x} & \tan( \arctan x ) &= x \end{align*}

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