# Definitions, simple applications, and graphs of trigonometric  (Page 7/7)

 Page 7 / 7

Let us look back at our values for $tan\theta$

 $\theta$ ${0}^{\circ }$ ${30}^{\circ }$ ${45}^{\circ }$ ${60}^{\circ }$ ${90}^{\circ }$ ${180}^{\circ }$ $tan\theta$ 0 $\frac{1}{\sqrt{3}}$ 1 $\sqrt{3}$ $\infty$ 0

Now that we have graphs for $sin\theta$ and $cos\theta$ , there is an easy way to visualise the tangent graph. Let us look back at our definitions of $sin\theta$ and $cos\theta$ for a right-angled triangle.

$\frac{sin\theta }{cos\theta }=\frac{\frac{\mathrm{opposite}}{\mathrm{hypotenuse}}}{\frac{\mathrm{adjacent}}{\mathrm{hypotenuse}}}=\frac{\mathrm{opposite}}{\mathrm{adjacent}}=tan\theta$

This is the first of an important set of equations called trigonometric identities . An identity is an equation, which holds true for any value which is put into it. In this case we have shown that

$tan\theta =\frac{sin\theta }{cos\theta }$

for any value of $\theta$ .

So we know that for values of $\theta$ for which $sin\theta =0$ , we must also have $tan\theta =0$ . Also, if $cos\theta =0$ our value of $tan\theta$ is undefined as we cannot divide by 0. The graph is shown in [link] . The dashed vertical lines are at the values of $\theta$ where $tan\theta$ is not defined.

## Functions of the form $y=atan\left(x\right)+q$

In the figure below is an example of a function of the form $y=atan\left(x\right)+q$ .

## Functions of the form $y=atan\left(\theta \right)+q$ :

1. On the same set of axes, plot the following graphs:
1. $a\left(\theta \right)=tan\theta -2$
2. $b\left(\theta \right)=tan\theta -1$
3. $c\left(\theta \right)=tan\theta$
4. $d\left(\theta \right)=tan\theta +1$
5. $e\left(\theta \right)=tan\theta +2$
Use your results to deduce the effect of $q$ .
2. On the same set of axes, plot the following graphs:
1. $f\left(\theta \right)=-2·tan\theta$
2. $g\left(\theta \right)=-1·tan\theta$
3. $h\left(\theta \right)=0·tan\theta$
4. $j\left(\theta \right)=1·tan\theta$
5. $k\left(\theta \right)=2·tan\theta$
Use your results to deduce the effect of $a$ .

You should have found that the value of $a$ affects the steepness of each of the branches. The larger the absolute magnitude of a , the quicker the branches approach their asymptotes, the values where they are not defined. Negative $\mathit{a}$ values switch the direction of the branches. You should have also found that the value of $q$ affects the vertical shift as for $sin\theta$ and $cos\theta$ . These different properties are summarised in [link] .

 $a>0$ $a<0$ $q>0$ $q<0$

## Domain and range

The domain of $f\left(\theta \right)=atan\left(\theta \right)+q$ is all the values of $\theta$ such that $cos\theta$ is not equal to 0. We have already seen that when $cos\theta =0$ , $tan\theta =\frac{sin\theta }{cos\theta }$ is undefined, as we have division by zero. We know that $cos\theta =0$ for all $\theta ={90}^{\circ }+{180}^{\circ }n$ , where $n$ is an integer. So the domain of $f\left(\theta \right)=atan\left(\theta \right)+q$ is all values of $\theta$ , except the values $\theta ={90}^{\circ }+{180}^{\circ }n$ .

The range of $f\left(\theta \right)=atan\theta +q$ is $\left\{f\left(\theta \right):f\left(\theta \right)\in \left(-\infty ,\infty \right)\right\}$ .

## Intercepts

The $y$ -intercept, ${y}_{int}$ , of $f\left(\theta \right)=atan\left(x\right)+q$ is again simply the value of $f\left(\theta \right)$ at $\theta ={0}^{\circ }$ .

$\begin{array}{ccc}\hfill {y}_{int}& =& f\left({0}^{\circ }\right)\hfill \\ & =& atan\left({0}^{\circ }\right)+q\hfill \\ & =& a\left(0\right)+q\hfill \\ & =& q\hfill \end{array}$

## Asymptotes

As $\theta$ approaches ${90}^{\circ }$ , $tan\theta$ approaches infinity. But as $\theta$ is undefined at ${90}^{\circ }$ , $\theta$ can only approach ${90}^{\circ }$ , but never equal it. Thus the $tan\theta$ curve gets closer and closer to the line $\theta ={90}^{\circ }$ , without ever touching it. Thus the line $\theta ={90}^{\circ }$ is an asymptote of $tan\theta$ . $tan\theta$ also has asymptotes at $\theta ={90}^{\circ }+{180}^{\circ }n$ , where $n$ is an integer.

## Graphs of trigonometric functions

1. Using your knowldge of the effects of $a$ and $q$ , sketch each of the following graphs, without using a table of values, for $\theta \in \left[{0}^{\circ };{360}^{\circ }\right]$
1. $y=2sin\theta$
2. $y=-4cos\theta$
3. $y=-2cos\theta +1$
4. $y=sin\theta -3$
5. $y=tan\theta -2$
6. $y=2cos\theta -1$
2. Give the equations of each of the following graphs:

The following presentation summarises what you have learnt in this chapter. Ignore the last slide.

## End of chapter exercises

1. Calculate the unknown lengths
2. In the triangle $PQR$ , $PR=20$  cm, $QR=22$  cm and $P\stackrel{^}{R}Q={30}^{\circ }$ . The perpendicular line from $P$ to $QR$ intersects $QR$ at $X$ . Calculate
1. the length $XR$ ,
2. the length $PX$ , and
3. the angle $Q\stackrel{^}{P}X$
3. A ladder of length 15 m is resting against a wall, the base of the ladder is 5 m from the wall. Find the angle between the wall and the ladder?
4. A ladder of length 25 m is resting against a wall, the ladder makes an angle ${37}^{\circ }$ to the wall. Find the distance between the wall and the base of the ladder?
5. In the following triangle find the angle $A\stackrel{^}{B}C$
6. In the following triangle find the length of side $CD$
7. $A\left(5;0\right)$ and $B\left(11;4\right)$ . Find the angle between the line through A and B and the x-axis.
8. $C\left(0;-13\right)$ and $D\left(-12;14\right)$ . Find the angle between the line through C and D and the y-axis.
9. A $5\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ ladder is placed $2\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ from the wall. What is the angle the ladder makes with the wall?
10. Given the points: E(5;0), F(6;2) and G(8;-2), find angle $F\stackrel{^}{E}G$ .
11. An isosceles triangle has sides $9\phantom{\rule{0.166667em}{0ex}}\mathrm{cm},\phantom{\rule{0.166667em}{0ex}}9\phantom{\rule{0.166667em}{0ex}}\mathrm{cm}$ and $2\phantom{\rule{0.166667em}{0ex}}\mathrm{cm}$ . Find the size of the smallest angle of the triangle.
12. A right-angled triangle has hypotenuse $13\phantom{\rule{0.166667em}{0ex}}\mathrm{mm}$ . Find the length of the other two sides if one of the angles of the triangle is ${50}^{\circ }$ .
13. One of the angles of a rhombus ( rhombus - A four-sided polygon, each of whose sides is of equal length) with perimeter $20\phantom{\rule{0.166667em}{0ex}}\mathrm{cm}$ is ${30}^{\circ }$ .
1. Find the sides of the rhombus.
2. Find the length of both diagonals.
14. Captain Hook was sailing towards a lighthouse with a height of $10\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ .
1. If the top of the lighthouse is $30\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ away, what is the angle of elevation of the boat to the nearest integer?
2. If the boat moves another $7\phantom{\rule{0.166667em}{0ex}}\mathrm{m}$ towards the lighthouse, what is the new angle of elevation of the boat to the nearest integer?
15. (Tricky) A triangle with angles ${40}^{\circ },\phantom{\rule{0.166667em}{0ex}}{40}^{\circ }$ and ${100}^{\circ }$ has a perimeter of $20\phantom{\rule{0.166667em}{0ex}}\mathrm{cm}$ . Find the length of each side of the triangle.

#### Questions & Answers

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20/(×-6^2)
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The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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Source:  OpenStax, Math 1508 (lecture) readings in precalculus. OpenStax CNX. Aug 24, 2011 Download for free at http://cnx.org/content/col11354/1.1
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