Q. 59 - Precalculus Enhanced with Graphing Utilities

Question

Find the exact values of the six trigonometric functions of the given angle. If any are not defined, say "not defined." Do not use a calculator.

$-135°$

Step-by-Step Solution

Answer

The exact values of the six trigonometric functions are $\sin \left(-135°\right)=-\frac{\sqrt{2}}{2}$, $\cos \left(-135°\right)=-\frac{\sqrt{2}}{2}$, $\tan \left(-135°\right)=1$, $\csc \left(-135°\right)=-\sqrt{2}$, $\sec \left(-135°\right)=-\sqrt{2}$ and $\cot \left(-135°\right)=1$.

Step 1. Determine a point corresponds to the given angle.

The given angle is $-135°=360°-135°=225°=\frac{5\pi }{4}$. This angle is multiple of $\frac{\pi }{4}$.

From the below figure, we see the point $\left(-\frac{\sqrt{2}}{2},-\frac{\sqrt{2}}{2}\right)$ corresponds to $\frac{5\pi }{4}$.

Step 2. Determine the exact value of sine function.

The exact value of the sine function is:

$$\begin{gathered} \sin \left(-135°\right)=y \\ \sin \left(-135°\right)=-\frac{\sqrt{2}}{2} \end{gathered}$$

Step 3. Determine the exact value of cosine function.

The exact value of the cosine function is:

$$\begin{gathered} \cos \left(-135°\right)=x \\ \cos \left(-135°\right)=-\frac{\sqrt{2}}{2} \end{gathered}$$

Step 4. Determine the exact value of tangent function.

The exact value of the tangent function is:

$\begin{array}{rcl}\tan \theta & =& \frac{\sin \theta }{\cos \theta }\\ \tan \left(-135°\right)& =& \frac{-\frac{\sqrt{2}}{2}}{-\frac{\sqrt{2}}{2}}\\ \tan \left(-135°\right)& =& 1\end{array}$

Step 5. Determine the exact value of cosecant function.

The exact value of the cosecant function is:

$\begin{array}{rcl}\csc \theta & =& \frac{1}{\sin \theta }\\ \csc \left(-135°\right)& =& \frac{1}{-\frac{\sqrt{2}}{2}}\\ \csc \left(-135°\right)& =& -\frac{2}{\sqrt{2}}\\ \csc \left(-135°\right)& =& -\sqrt{2}\end{array}$

Step 6. Determine the exact value of secant function.

The exact value of the secant function is:

$\begin{array}{rcl}\sec \theta & =& \frac{1}{\cos \theta }\\ \sec \left(-135°\right)& =& \frac{1}{-\frac{\sqrt{2}}{2}}\\ \sec \left(-135°\right)& =& -\frac{2}{\sqrt{2}}\\ \sec \left(-135°\right)& =& -\sqrt{2}\end{array}$

Step 7. Determine the exact value of cotangent function.

The exact value of the cotangent function is:

$\begin{array}{rcl}\cot \theta & =& \frac{1}{\tan \theta }\\ \cot \left(-135°\right)& =& \frac{1}{1}\\ \cot \left(-135°\right)& =& 1\end{array}$