SAT Mathematics : Circles

Study concepts, example questions & explanations for SAT Mathematics

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Example Questions

Example Question #1 : Using Radians

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(\pi-6)}{\pi+6}

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

\displaystyle x=\frac{\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 #2 : Using Radians

Give \displaystyle x in radians:

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

Possible Answers:

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

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

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

\displaystyle x=\frac{2\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 : Using Radians

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

Possible Answers:

\displaystyle 180^\circ

\displaystyle 130^\circ

\displaystyle 150^\circ

\displaystyle 210^\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

Example Question #1 : Circles

Change the following expression to degrees:

\displaystyle \frac{\frac{\pi}{3}-\frac{\pi}{6}}{\frac{1}{3}+\frac{1}{6}}

Possible Answers:

\displaystyle 60^{\circ}

\displaystyle 120^{\circ}

\displaystyle 90^{\circ}

\displaystyle 75^{\circ}

Correct answer:

\displaystyle 60^{\circ}

Explanation:

First, we need to simplify the expression:

\displaystyle \frac{\frac{\pi}{3}-\frac{\pi}{6}}{\frac{1}{3}+\frac{1}{6}}=\frac{\frac{2\pi-\pi}{6}}{\frac{2+1}{6}}=\frac{\pi}{3}

 

Now multiply by \displaystyle \frac{180^{\circ}}{\pi}:

\displaystyle \frac{\pi}{3}\times \frac{180^{\circ}}{\pi}=\frac{180^{\circ}}{3}=60^{\circ}

Example Question #5 : Using Radians

Convert the following expression to radians:

\displaystyle \frac{45^{\circ}+30^{\circ}}{45-30}

Possible Answers:

\displaystyle \frac{\pi}{30}

\displaystyle \frac{\pi}{36}

\displaystyle \frac{7\pi}{36}

\displaystyle \frac{5\pi}{36}

Correct answer:

\displaystyle \frac{\pi}{36}

Explanation:

First, we need to simplify the expression:

\displaystyle \frac{45^{\circ}+30^{\circ}}{45-30}=\frac{75^{\circ}}{15}=5^{\circ}

 

In order to change degrees to radians, we need to multiply by \displaystyle \frac{\pi}{180^{\circ}}:

\displaystyle 5^{\circ}\times \frac{\pi}{180^{\circ}}=\frac{\pi}{36}

Example Question #6 : Using Radians

Simplify and give the following expression in degrees: 

\displaystyle \frac{\frac{2\pi}{7}+\frac{\pi}{14}}{\frac{1}{7}}

Possible Answers:

\displaystyle 480^{\circ}

\displaystyle 450^{\circ}

\displaystyle 350^{\circ}

\displaystyle 420^{\circ}

Correct answer:

\displaystyle 450^{\circ}

Explanation:

First, we need to simplify the expression:

\displaystyle \frac{\frac{2\pi}{7}+\frac{\pi}{14}}{\frac{1}{7}}=\frac{\frac{4\pi+\pi}{14}}{\frac{1}{7}}=\frac{5\pi}{2}

Then multiply by \displaystyle \frac{180^{\circ}}{\pi}:

\displaystyle \frac{5\pi}{2}\times \frac{180^{\circ}}{\pi}=\frac{5\times 180^{\circ}}{2}=450^{\circ}

Example Question #7 : Using Radians

Convert \displaystyle \frac{7\pi}{4} radians into degrees.

Possible Answers:

\displaystyle 285$^{\circ}$

\displaystyle 315$^{\circ}$

\displaystyle 345$^{\circ}$

\displaystyle 225$^{\circ}$

Correct answer:

\displaystyle 315$^{\circ}$

Explanation:

Recall the definition of "radians" derived from the unit circle:

\displaystyle 180$^{\circ}$ = \pi rad

The quantity of radians given in the problem is \displaystyle \frac{7\pi}{4}. All that is required to convert this measure into degrees is to denote the unknown angle measure in degrees by \displaystyle \Theta and set up a proportion equation using the aforementioned definition relating radians to degrees:

\displaystyle \frac{180^{\circ}}{\Theta} = \frac{\pi rad}{\frac{7\pi}{4} rad}

Cross-multiply the denominators in these fractions to obtain:

\displaystyle 1260^{\circ}\pi rad=4\Theta\pi rad

or

\displaystyle 315^{\circ}\pi rad =\Theta\pi rad.

Canceling like terms in these equations yields

\displaystyle \Theta = 315^{\circ}

Hence, the correct angle measure of \displaystyle \frac{7\pi}{4} in degrees is \displaystyle 315^{\circ}.

Example Question #8 : Using Radians

\displaystyle \frac{37\pi}{18} radians is equivalent to how many degrees?

Possible Answers:

\displaystyle 350^\circ

\displaystyle 10^\circ

\displaystyle 185^\circ

\displaystyle 370^\circ

Correct answer:

\displaystyle 370^\circ

Explanation:

1 radian is equal to \displaystyle \frac{180}{\pi} degrees. Using this conversion factor,

\displaystyle \frac{37\pi}{18}\times\frac{180}{\pi}=37\times10=370.

Example Question #9 : Using Radians

Simplify your answer.

Convert \displaystyle 45^{\circ} to radians:

Possible Answers:

\displaystyle \frac{1 }{4}

\displaystyle \frac{\pi }{4}

\displaystyle \frac{\pi }{2}

\displaystyle \frac{\pi }{8}

Correct answer:

\displaystyle \frac{\pi }{4}

Explanation:

We know that:

\displaystyle 360^{\circ}=2\pi Radians

since the giving angle was in degrees then we multiply

\displaystyle 45^{\circ}*\frac{\pi}{180^{\circ}}= \frac{\pi}{4}

Example Question #10 : Using Radians

Give your answer in terms of \displaystyle \pi.

Convert \displaystyle 165^{\circ}  to radians:

Possible Answers:

\displaystyle \frac{12\pi}{11}

\displaystyle \frac{11\pi}{6}

\displaystyle \frac{11\pi}{24}

\displaystyle \frac{11\pi}{12}

Correct answer:

\displaystyle \frac{11\pi}{12}

Explanation:

To convert degrees to radians, we need to multiply the given degree by \displaystyle \frac{\pi}{180^{\circ}}.

\displaystyle 165*^{\circ}\frac{\pi}{180^{\circ}}=\frac{165\pi}{180}

To simplify, we get:

\displaystyle \frac{11\pi}{12}

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