# 5.5 Graphs of trig functions  (Page 3/3)

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 $\theta$ ${0}^{\circ }$ ${30}^{\circ }$ ${45}^{\circ }$ ${60}^{\circ }$ ${90}^{\circ }$ ${180}^{\circ }$ $tan\theta$ 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.

## Summary

• We can define three trigonometric functions for right angled triangles: sine (sin), cosine (cos) and tangent (tan).
• Each of these functions have a reciprocal: cosecant (cosec), secant (sec) and cotangent (cot).
• We can use the principles of solving equations and the trigonometric functions to help us solve simple trigonometric equations.
• We can solve problems in two dimensions that involve right angled triangles.
• For some special angles, we can easily find the values of sin, cos and tan.
• We can extend the definitions of the trigonometric functions to any angle.
• Trigonometry is used to help us solve problems in 2-dimensions, such as finding the height of a building.
• We can draw graphs for sin, cos and tan

## 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.

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