Trigonometry : Radians and Conversions

Study concepts, example questions & explanations for Trigonometry

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Example Questions

Example Question #1 : Radians And Conversions

Change \(\displaystyle \frac{4\pi }{3}\) angle to degrees.

Possible Answers:

\(\displaystyle 300^{\circ}\)

\(\displaystyle 150^{\circ}\)

\(\displaystyle 240^{\circ}\)

\(\displaystyle 180^{\circ}\)

Correct answer:

\(\displaystyle 240^{\circ}\)

Explanation:

In order to change an angle into degrees, you must multiply the radian by \(\displaystyle \frac{180^{\circ}}{\pi}\).

Therefore, to solve:

\(\displaystyle \frac{4\pi }{3} \cdot \frac{180^{\circ} }{\pi} = 240^{\circ}\)

Example Question #2 : Radians And Conversions

Give \(\displaystyle x\) in radians:

 

\(\displaystyle \frac{x+\pi}{x-\pi}=30^{\circ}\)

Possible Answers:

\(\displaystyle x=\frac{\pi(\pi+6)}{\pi-6}\)

\(\displaystyle x=\frac{\pi+6}{\pi-6}\)

\(\displaystyle x=\frac{\pi(\pi+8)}{\pi-8}\)

\(\displaystyle x=\pi\)

\(\displaystyle x=\frac{\pi(\pi-6)}{\pi+6}\)

Correct answer:

\(\displaystyle x=\frac{\pi(\pi+6)}{\pi-6}\)

Explanation:

First we need to convert degrees to radians by multiplying by \(\displaystyle \frac{\pi}{180^{\circ}}\):

 

\(\displaystyle 30^{\circ}\times \frac{\pi}{180^{\circ}}=\frac{\pi}{6}\)

 

Now we can write:

 

\(\displaystyle \frac{x+\pi}{x-\pi}=\frac{\pi}{6}\Rightarrow 6(x+\pi)=\pi(x-\pi)\)

\(\displaystyle \Rightarrow 6x+6\pi=x\pi-\pi^2\Rightarrow 6x-x\pi=-6\pi-\pi^2\)

\(\displaystyle \Rightarrow x(6-\pi)=-\pi(\pi+6)\)

\(\displaystyle \Rightarrow x=\frac{-\pi(\pi+6)}{6-\pi}=\frac{\pi(\pi+6)}{\pi-6}\)

 

Example Question #1 : Radians And Conversions

Give \(\displaystyle x\) in radians:

 

\(\displaystyle x-\pi=120^{\circ}\)

Possible Answers:

\(\displaystyle x=\frac{5\pi}{3}\)

\(\displaystyle x=\frac{\pi}{3}\)

\(\displaystyle x=\frac{2\pi}{3}\)

\(\displaystyle x=\frac{4\pi}{3}\)

\(\displaystyle x=\frac{5\pi}{6}\)

Correct answer:

\(\displaystyle x=\frac{5\pi}{3}\)

Explanation:

First we need to convert degrees to radians by multiplying by \(\displaystyle \frac{\pi}{180^{\circ}}\):

 

\(\displaystyle 120^{\circ}\times \frac{\pi}{180^{\circ}}=\frac{2\pi}{3}\)

 

Now we can write:

 

\(\displaystyle x-\pi=120^{\circ}\Rightarrow x-\pi=\frac{2\pi}{3}\Rightarrow x=\frac{2\pi}{3}+\pi=\frac{5\pi}{3}\)

Example Question #2 : Radians And Conversions

Give \(\displaystyle x\) in degrees:

 

\(\displaystyle \frac{3x+6\pi}{2}=4\pi\)

Possible Answers:

\(\displaystyle x=150^{\circ}\)

\(\displaystyle x=210^{\circ}\)

\(\displaystyle x=180^{\circ}\)

\(\displaystyle x=30^{\circ}\)

\(\displaystyle x=120^{\circ}\)

Correct answer:

\(\displaystyle x=120^{\circ}\)

Explanation:

First we can find \(\displaystyle x\) in radians:

 

\(\displaystyle \frac{3x+6\pi}{2}=4\pi\Rightarrow 3x+6\pi=2\times 4\pi\)

\(\displaystyle \Rightarrow 3x+6\pi=8\pi\Rightarrow 3x=8\pi-6\pi\)

\(\displaystyle \Rightarrow 3x=2\pi\Rightarrow x=\frac{2\pi}{3}\)

 

To change radians to degrees we need to multiply radians by \(\displaystyle \frac{180^{\circ}}{\pi}\). So we can write:

 

\(\displaystyle x=\frac{2\pi}{3}\Rightarrow x=\frac{2\pi}{3}\times \frac{180^{\circ}}{\pi}=120^{\circ}\)

 

Example Question #4 : Radians

Give \(\displaystyle x\) in degrees:

 

\(\displaystyle \frac{4x-\pi}{2}=3\pi\)

Possible Answers:

\(\displaystyle x=345^{\circ}\)

\(\displaystyle x=305^{\circ}\)

\(\displaystyle x=315^{\circ}\)

\(\displaystyle x=415^{\circ}\)

\(\displaystyle x=300^{\circ}\)

Correct answer:

\(\displaystyle x=315^{\circ}\)

Explanation:

First we can find \(\displaystyle x\) in radians:

 

\(\displaystyle \frac{4x-\pi}{2}=3\pi\Rightarrow 4x-\pi=2\times 3\pi\)

\(\displaystyle \Rightarrow 4x-\pi=6\pi\Rightarrow 4x=6\pi+\pi\)

\(\displaystyle \Rightarrow 4x=7\pi\Rightarrow x=\frac{7\pi}{4}\)

To change radians to degrees we need to multiply radians by \(\displaystyle \frac{180^{\circ}}{\pi}\). So we can write:

 

\(\displaystyle x=\frac{7\pi}{4}\Rightarrow x=\frac{7\pi}{4}\times \frac{180^{\circ}}{\pi}=315^{\circ}\)

Example Question #3 : Radians And Conversions

Give \(\displaystyle x\) in radians:

 

\(\displaystyle \frac{72^{\circ}-\pi}{2x-\pi}=3\)

Possible Answers:

\(\displaystyle x=\frac{2\pi}{5}\)

\(\displaystyle x=\frac{\pi}{5}\)

\(\displaystyle x=\frac{4\pi}{5}\)

\(\displaystyle x=\frac{2\pi}{3}\)

\(\displaystyle x=\frac{3\pi}{5}\)

Correct answer:

\(\displaystyle x=\frac{2\pi}{5}\)

Explanation:

First we need to convert \(\displaystyle 72^{\circ}\) to radians by multiplying by \(\displaystyle \frac{\pi}{180^{\circ}}\):

 

\(\displaystyle 72^{\circ}\times \frac{\pi}{180^{\circ}}=\frac{2\pi}{5}\)

 

Now we can solve the following equation for \(\displaystyle x\):

 

\(\displaystyle \frac{72^{\circ}-\pi}{2x-\pi}=3\Rightarrow \frac{\frac{2\pi}{5}-\pi}{2x-\pi}=3\)

\(\displaystyle \Rightarrow {\frac{2\pi}{5}-\pi}=3(2x-\pi)\Rightarrow \frac{2\pi-5\pi}{5}=6x-3\pi\)

\(\displaystyle \Rightarrow -3\pi=30x-15\pi\Rightarrow 30x=12\pi\)

\(\displaystyle \Rightarrow x=\frac{12\pi}{30}\Rightarrow x=\frac{2\pi}{5}\)

Example Question #3 : Radians And Conversions

Give \(\displaystyle x\) in degrees:

 

\(\displaystyle \frac{\frac{\pi}{2}-x}{\frac{\pi}{2}+x}=\frac{1}{2}\)

Possible Answers:

\(\displaystyle x=60^{\circ}\)

\(\displaystyle x=90^{\circ}\)

\(\displaystyle x=30^{\circ}\)

\(\displaystyle x=36^{\circ}\)

\(\displaystyle x=40^{\circ}\)

Correct answer:

\(\displaystyle x=30^{\circ}\)

Explanation:

First we can find \(\displaystyle x\) in radians:

 

\(\displaystyle \frac{\frac{\pi}{2}-x}{\frac{\pi}{2}+x}=\frac{1}{2}\Rightarrow 2(\frac{\pi}{2}-x)=1(\frac{\pi}{2}+x)\)

\(\displaystyle \Rightarrow \pi-2x=\frac{\pi}{2}+x\Rightarrow \pi-\frac{\pi}{2}=3x\)

\(\displaystyle \Rightarrow 3x=\frac{\pi}{2}\Rightarrow x=\frac{\pi}{6}\)

To change radians to degrees we need to multiply radians by \(\displaystyle \frac{180^{\circ}}{\pi}\). So we can write:

 

\(\displaystyle x=\frac{\pi}{6}\Rightarrow x=\frac{\pi}{6}\times \frac{180^{\circ}}{\pi}=30^{\circ}\)

Example Question #3 : Radians And Conversions

Convert the angle \(\displaystyle \frac{7\pi }{3}\) into degrees.

Possible Answers:

\(\displaystyle 420^{\circ}\)

\(\displaystyle 315^{\circ}\)

\(\displaystyle 840^{\circ}\)

\(\displaystyle 500^{\circ}\)

\(\displaystyle 210^{\circ}\)

Correct answer:

\(\displaystyle 420^{\circ}\)

Explanation:

To convert radians to degrees, use the conversion \(\displaystyle \frac{180^{\circ}}{\pi }\).

In this case:

\(\displaystyle (\frac{7\pi }{3})*(\frac{180^{\circ}}{\pi }) = 420^{\circ}\)

Example Question #21 : Unit Circle And Radians

How many radians are in \(\displaystyle 300^\circ\)?

Possible Answers:

\(\displaystyle \frac{5\pi}{3}\)

\(\displaystyle \frac{7\pi}{3}\)

\(\displaystyle \frac{3\pi}{5}\)

\(\displaystyle \pi\sqrt{3}\)

\(\displaystyle \frac{2\pi}{3}\)

Correct answer:

\(\displaystyle \frac{5\pi}{3}\)

Explanation:

Since\(\displaystyle 180^\circ=\pi\ \text{radians}\), we can solve by setting up a proportion:

\(\displaystyle \frac{300^\circ}{x}=\frac{180^\circ}{\pi}\)

Cross-multiply and solve.

\(\displaystyle 300^\circ*\pi=180^\circ*x\)

\(\displaystyle \frac{300^\circ}{180^\circ}\pi=x\)

\(\displaystyle \frac{5\pi}{3}=x\)

Example Question #4 : Radians And Conversions

How many degrees are in \(\displaystyle \frac{5\pi}{6}\) radians?

Possible Answers:

\(\displaystyle 150^\circ\)

\(\displaystyle 130^\circ\)

\(\displaystyle 210^\circ\)

\(\displaystyle 180^\circ\)

\(\displaystyle 160^\circ\)

Correct answer:

\(\displaystyle 150^\circ\)

Explanation:

Since \(\displaystyle 180^\circ=\pi\ \text{radians}\), we can solve by setting up a proportion:

\(\displaystyle \frac{x}{\frac{5\pi}{6}}=\frac{180^\circ}{\pi}\)

Cross multiply and solve.

\(\displaystyle 180^\circ*\frac{5\pi}{6}=x*\pi\)

\(\displaystyle \frac{900\pi}{6}=x*\pi\)

\(\displaystyle 150\pi=x*\pi\)

\(\displaystyle \frac{150\pi}{\pi}=x\)

\(\displaystyle 150^\circ=x\)

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