# Trigonometric Ratios in Quadrants by a Unit Circle

## Circles with Centre $(0,0)$

Consider a circle with centre $(0,0)$ and radius $r$ units. Suppose $(x,y)$ is any point on this circle.
Using this distance formula;
\begin{align} \displaystyle \sqrt{(x-0)^2+(y-0)^2} &= r \\ \therefore x^2+y^2 &= r^2 \end{align}
$x^2+y^2 = r^2$ is the equation of a circle with centre $(0,0)$ and radius $r$.
The equation of the unit circle is $x^2+y^2=1$.
\begin{align} \dfrac{x}{r} &= \cos \theta &\therefore x &= r \cos \theta \\ \dfrac{y}{r} &= \sin \theta &\therefore y &= r \sin \theta \end{align}

In the first quadrant, the coordinates of $A$ are $(\cos \theta,\sin \theta)$.
$\dfrac{\sin \theta}{\cos \theta} = \tan \theta$

In the second quadrant, the coordinates of $B$ are $(-\cos \theta,\sin \theta)$.
$\dfrac{\sin \theta}{-\cos \theta} = -\tan \theta$

In the third quadrant, the coordinates of $C$ is $(-\cos \theta,-\sin \theta)$.
$\dfrac{-\sin \theta}{-\cos \theta} = \tan \theta$

In the fourth quadrant, the coordinates of $D$ are $(\cos \theta,-\sin \theta)$.
$\dfrac{\sin \theta}{-\cos \theta} = -\tan \theta$

In summary, the signs of $\sin \theta$, $\cos \theta$ and $\tan \theta$ in each quadrant are;

From the investigation above, you should notice that:

• $\sin \theta$, $\cos \theta$ and $\tan \theta$ are positive in the first quadrant
• Only $\sin \theta$ is positive in the second quadrant
• Only $\tan \theta$ is positive in the third quadrant
• Only $\cos \theta$ is positive in the fourth quadrant

We can use a letter to show which trigonometric ratios are positive in each quadrant. The A stands for $\text{all}$ of the ratios. You might like to remember them using;
“All Silly Turtles Crawl” or “All Stations To Central”.

## Periodicity of Trigonometric Ratios

Since there are $2\pi$ radians or $360^{\circ}$ in a full revolution, if we add any integer multiple of $2 \pi$ to $\theta$, in radians, then the position of the point on the unit circle is unchanged.
\begin{aligned} \cos \theta &= \cos (\theta + 2 \pi) \\ &= \cos (\theta + 4 \pi) \\ &= \cos (\theta + 6 \pi) \\ &= \cos (\theta + 8 \pi) &\cdots \end{aligned}
For example, $\cos 5\pi = \cos 3\pi = \cos \pi$.
\begin{aligned} \sin \theta &= \sin (\theta + 2 \pi) \\ &= \sin (\theta + 4 \pi) \\ &= \sin (\theta + 6 \pi) \\ &= \sin (\theta + 8 \pi) \\ &\cdots \end{aligned}
For example, $\sin \dfrac{9 \pi}{2} = \sin \dfrac{5 \pi}{2} = \sin \dfrac{\pi}{2}$.

Particularly, if we add any integers of $\pi$ to $\theta$, in radians, then the tangent value remains unchanged.
\begin{aligned} \tan \theta &= \tan (\theta + \pi) \\ &= \tan (\theta + 2 \pi) \\ &= \tan (\theta + 3 \pi) \\ &= \tan (\theta + 4 \pi) \\ &\cdots \end{aligned}
For example, $\tan \dfrac{7 \pi}{3} = \tan \dfrac{4 \pi}{3} = \tan \dfrac{\pi}{3}$.

## Example 1

State the coordinates of $A$, correct to three significant figures.

\begin{align} \displaystyle \cos 56^{\circ} &= 0.5591 \cdots = 0.559 \\ \sin 56^{\circ} &= 0.8290 \cdots = 0.829 \\ \therefore (0.599,0.829) \end{align}

## Example 2

State the coordinates of $B$, correct to three significant figures.

\begin{align} \displaystyle \cos 145^{\circ} &= -0.8191 \cdots = -0.819 \\ \sin 145^{\circ} &= 0.5735 \cdots = 0.574 \\ \therefore (-0.819,0.574) \end{align}

## Example 3

State the coordinates of $C$, correct to three significant figures.

\begin{align} \displaystyle \cos 235^{\circ} &= -0.5735 \cdots = -0.574 \\ \sin 235^{\circ} &= -0.8191 \cdots = -0.819 \\ \therefore (-0.574,-0.819) \end{align}

## Example 4

Use $\sin {(180^{\circ}-\theta)} = \sin \theta$, find the obtuse angle with the same sine as $45^{\circ}$.

\begin{align} \displaystyle \sin 45^{\circ} &= \sin {(180^{\circ}-45^{\circ})} \\ &= \sin{135^{\circ}} \end{align}

## Example 5

Use $\cos {(\pi-\theta)} = \cos \theta$, find the obtuse angle with the same cosine as $\dfrac{\pi}{3}$.

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