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# Quotient Identities

## Tangent equals sine divided by cosine.

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Quotient Identities

You are working in math class one day when your friend leans over and asks you what you got for the sine and cosine of a particular angle.

"I got 12\begin{align*}\frac{1}{2}\end{align*} for the sine, and 32\begin{align*}\frac{\sqrt{3}}{2}\end{align*} for the cosine. Why?" you ask.

"It looks like I'm supposed to calculate the tangent function for the same angle you just did, but I can't remember the relationship for tangent. What should I do?" he says.

Do you know how you can help your friend find the answer, even if both you and he don't remember the relationship for tangent?

### Quotient Identities

The definitions of the trig functions led us to the reciprocal identities, which can be seen in the Concept about that topic. They also lead us to another set of identities, the quotient identities.

Consider first the sine, cosine, and tangent functions. For angles of rotation (not necessarily in the unit circle) these functions are defined as follows:

sinθcosθtanθ=yr=xr=yx\begin{align*}\sin \theta & = \frac{y}{r}\\ \cos \theta & = \frac{x}{r}\\ \tan \theta & = \frac{y}{x}\end{align*}

Given these definitions, we can show that tanθ=sinθcosθ\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta}\end{align*}, as long as cosθ0\begin{align*}\cos \theta \ne 0\end{align*}:

sinθcosθ=yrxr=yr×rx=yx=tanθ.\begin{align*}\frac{\sin \theta}{\cos \theta} = \frac{\frac{y}{r}}{\frac{x}{r}} = \frac{y}{r} \times \frac{r}{x} = \frac{y}{x} = \tan \theta.\end{align*}

The equation tanθ=sinθcosθ\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta}\end{align*} is therefore an identity that we can use to find the value of the tangent function, given the value of the sine and cosine.

#### Find the value of tanθ\begin{align*}\tan \theta\end{align*}?

If cosθ=513\begin{align*}\cos \theta = \frac{5}{13}\end{align*} and sinθ=1213\begin{align*}\sin \theta = \frac{12}{13}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?

tanθ=125\begin{align*}\tan \theta = \frac{12}{5}\end{align*}

tanθ=sinθcosθ=1213513=1213×135=125\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{\frac{12}{13}}{\frac{5}{13}} = \frac{12}{13} \times \frac{13}{5} = \frac{12} {5}\end{align*}

#### Show that cotθ=cosθsinθ\begin{align*}\cot \theta = \frac{\cos \theta}{\sin \theta}\end{align*}

cosθsinθ=xryr=xr×ry=xy=cotθ\begin{align*}\frac{\cos \theta}{\sin \theta} = \frac{\frac{x}{r}}{\frac{y}{r}} = \frac{x}{r} \times \frac{r}{y} = \frac{x}{y} = \cot \theta\end{align*}

#### What is the value of cotθ\begin{align*}\cot \theta\end{align*}?

If cosθ=725\begin{align*}\cos \theta = \frac{7}{25}\end{align*} and sinθ=2425\begin{align*}\sin \theta = \frac{24}{25}\end{align*}, what is the value of cotθ\begin{align*}\cot \theta\end{align*}?

cotθ=724\begin{align*}\cot \theta = \frac{7}{24}\end{align*}

cotθ=cosθsinθ=7252425=725×2524=724\begin{align*}\cot \theta = \frac{\cos \theta}{\sin \theta} = \frac{\frac{7}{25}}{\frac{24}{25}} = \frac{7}{25} \times \frac{25}{24} = \frac{7} {24}\end{align*}

### Examples

#### Example 1

Since you now know that:

tanθ=sinθcosθ\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta}\end{align*}

you can use this knowledge to help your friend with the sine and cosine values you measured for yourself earlier:

tanθ=sinθcosθ=1232=13\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{\frac{1}{2}}{\frac{\sqrt{3}}{2}} = \frac{1}{\sqrt{3}}\end{align*}

#### Example 2

If cosθ=17145\begin{align*}\cos \theta = \frac{17}{145}\end{align*} and sinθ=144145\begin{align*}\sin \theta = \frac{144}{145}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?

tanθ=14417\begin{align*}\tan \theta = \frac{144}{17}\end{align*}. We can see this from the relationship for the tangent function:

tanθ=sinθcosθ=14414517145=144145×14517=14417\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{\frac{144}{145}}{\frac{17}{145}} = \frac{144}{145} \times \frac{145}{17} = \frac{144} {17}\end{align*}

#### Example 3

If sinθ=6365\begin{align*}\sin \theta = \frac{63}{65}\end{align*} and cosθ=1665\begin{align*}\cos \theta = \frac{16}{65}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?

tanθ=6316\begin{align*}\tan \theta = \frac{63}{16}\end{align*}. We can see this from the relationship for the tangent function:

tanθ=sinθcosθ=63651665=6365×6516=6316\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta} = \frac{\frac{63}{65}}{\frac{16}{65}} = \frac{63}{65} \times \frac{65}{16} = \frac{63} {16}\end{align*}

#### Example 4

If tanθ=409\begin{align*}\tan \theta = \frac{40}{9}\end{align*} and cosθ=941\begin{align*}\cos \theta = \frac{9}{41}\end{align*}, what is the value of sinθ\begin{align*}\sin \theta\end{align*}?

sinθ=4041\begin{align*}\sin \theta = \frac{40}{41}\end{align*}. We can see this from the relationship for the tangent function:

tanθ=sinθcosθsinθ=(tanθ)(cosθ)sinθ=409×941sinθ=4041\begin{align*}\tan \theta = \frac{\sin \theta}{\cos \theta}\\ \sin \theta = (\tan \theta)(\cos \theta)\\ \sin \theta = \frac{40}{9} \times \frac{9}{41}\\ \sin \theta = \frac{40}{41} \end{align*}

### Review

Fill in each blank with a trigonometric function.

1. tanθ=sinθ?\begin{align*}\tan \theta= \frac{\sin \theta}{?}\end{align*}
2. cosθ=sinθ?\begin{align*}\cos \theta= \frac{\sin \theta}{?}\end{align*}
3. cotθ=?sinθ\begin{align*}\cot \theta= \frac{?}{\sin \theta}\end{align*}
4. cosθ=(cotθ)(?)\begin{align*}\cos \theta= (\cot \theta ) \cdot (?) \end{align*}
5. If cosθ=513\begin{align*}\cos \theta = \frac{5}{13}\end{align*} and sinθ=113\begin{align*}\sin \theta = \frac{1}{13}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
6. If sinθ=35\begin{align*}\sin \theta = \frac{3}{5}\end{align*} and cosθ=45\begin{align*}\cos \theta = \frac{4}{5}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
7. If cosθ=725\begin{align*}\cos \theta = \frac{7}{25}\end{align*} and sinθ=2425\begin{align*}\sin \theta = \frac{24}{25}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
8. If sinθ=1237\begin{align*}\sin \theta = \frac{12}{37}\end{align*} and cosθ=3537\begin{align*}\cos \theta = \frac{35}{37}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
9. If cosθ=2029\begin{align*}\cos \theta = \frac{20}{29}\end{align*} and sinθ=2129\begin{align*}\sin \theta = \frac{21}{29}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
10. If sinθ=3989\begin{align*}\sin \theta = \frac{39}{89}\end{align*} and cosθ=8089\begin{align*}\cos \theta = \frac{80}{89}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
11. If cosθ=4873\begin{align*}\cos \theta = \frac{48}{73}\end{align*} and sinθ=5573\begin{align*}\sin \theta = \frac{55}{73}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
12. If sinθ=6597\begin{align*}\sin \theta = \frac{65}{97}\end{align*} and cosθ=7297\begin{align*}\cos \theta = \frac{72}{97}\end{align*}, what is the value of tanθ\begin{align*}\tan \theta\end{align*}?
13. If cosθ=12\begin{align*}\cos \theta = \frac{1}{2}\end{align*} and cotθ=33\begin{align*}\cot \theta = \frac{\sqrt{3}}{3}\end{align*}, what is the value of sinθ\begin{align*}\sin \theta\end{align*}?
14. If tanθ=0\begin{align*}\tan \theta = 0\end{align*} and cosθ=1\begin{align*}\cos \theta = -1\end{align*}, what is the value of sinθ\begin{align*}\sin \theta\end{align*}?
15. If cotθ=1\begin{align*}\cot \theta = -1\end{align*} and sinθ=22\begin{align*}\sin \theta = -\frac{\sqrt{2}}{2}\end{align*}, what is the value of cosθ\begin{align*}\cos \theta\end{align*}?

To see the Review answers, open this PDF file and look for section 1.23.

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### Vocabulary Language: English

Quotient Identity

The quotient identity is an identity relating the tangent of an angle to the sine of the angle divided by the cosine of the angle.