We're used to measuring angles in degrees. However, angles can also be measured in radians (sometimes written as " \text{rad} "). One radian is the angle formed at the center of a circle by an arc whose length is equal to the radius of the circle.



To find the number of radians in a circle, we divide the circumference of the circle by the length of a radius. We always get the same result:

\dfrac{C}{r} = \dfrac{2 \pi r}{r} = 2\pi

So, we have that

  • 360^\circ is equivalent to 2\pi radians, and

  • 180^\circ is equivalent to \pi radians.

As a consequence,

  • to convert degrees to radians, we must multiply by \dfrac{\pi}{180^\circ}, and

  • to convert radians to degrees, multiply by \dfrac{180^\circ}{\pi}.

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An angle of 150^\circ is equal to how many radians?

EXPLANATION

To convert the measure of an angle in degrees to an equivalent measure in radians, we multiply the measure in degrees by \dfrac{\pi}{180^\circ}. This gives

\begin{align*} 150^\circ \cdot \left(\dfrac {\pi} {180^\circ} \right) &=\dfrac{150^\circ}{1} \cdot \left(\dfrac {\pi} {180^\circ} \right) \\[5pt] &=\dfrac{150^\circ}{180^\circ} \cdot \left(\dfrac {\pi} {1} \right) \\[5pt] &=\dfrac{150}{180} \cdot \left(\dfrac {\pi} {1} \right) \\[5pt] &=\dfrac{15}{18} \cdot \left(\dfrac {\pi} {1} \right) \\[5pt] &=\dfrac{5}{6} \cdot \left(\dfrac {\pi} {1} \right) \\[5pt] &= \dfrac {5\pi} {6} . \end{align*}

Therefore, 150^\circ is equivalent to \dfrac {5\pi} {6} radians.

Notice that when we divide 150^\circ by 180^\circ, we can drop the degree (\phantom{.}^\circ) symbol.

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An angle of $40^\circ$ is equal to how many radians?

a
$\dfrac {7\pi} {2}$
b
$\dfrac {\pi} {7}$
c
$\dfrac {5\pi} {4}$
d
$\dfrac {5\pi} {7}$
e
$\dfrac {2\pi} {9}$

An angle of $135^\circ$ is equal to

a
b
c
d
e

One-half of a full rotation is equal to how many radians?

a
$2\pi$ radians
b
$\pi$ radians
c
$4\pi$ radians
d
$\dfrac {\pi} 4$ radians
e
$\dfrac {\pi} 2$ radians

What is the angle of \dfrac{2 \pi} 3 radians in degrees?

EXPLANATION

To convert the measure of an angle in radians to an equivalent measure in degrees, we multiply the measure in radians by \dfrac{180^\circ}{\pi}. This gives

\begin{align*} \require{cancel} \left(\dfrac{2 \pi} 3\right) \cdot \left(\dfrac {180^\circ} {\pi}\right) &= \dfrac {2\cdot180^\circ\cdot\pi} {3\cdot\pi} \\[5pt] &= \dfrac {360^\circ\pi} {3\pi} \\[5pt] &= \dfrac {360^\circ\cancel{\pi}} {3\cancel{\pi}} \\[5pt] &= \dfrac {360^\circ} {3} \\[5pt] &= 120^\circ. \end{align*}

Therefore, \dfrac{2 \pi} 3 is equivalent to 120^\circ.

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An angle of $\dfrac{\pi}{8}$ radians is equal to

a
b
c
d
e

What is the angle of $2 \pi $ radians in degrees?

a
$90^\circ$
b
$180^\circ$
c
$160^\circ$
d
$150^\circ$
e
$360^\circ$

What is 0.78 radians in degrees to the nearest degree?

EXPLANATION

To convert the measure of an angle in radians to an equivalent measure in degrees, we multiply the measure in radians by \dfrac{180^\circ}{\pi}.

Evaluating this using a calculator, we get

0.78 \cdot \dfrac{180^\circ}{\pi} \approx 44.691^\circ, rounded to 3 decimal places. Therefore, 0.78 radians is equal to 45^\circ to the nearest degree.

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What is $0.957$ radians in degrees to the nearest degree?

a
$55^\circ$
b
$102^\circ$
c
$86^\circ$
d
$47^\circ$
e
$36^\circ$

Rounded to the nearest degree, an angle of $\dfrac{6\pi}{13}$ radians is equal to

a
b
c
d
e

Rounding to one decimal place, what is 30.2^\circ measured in radians?

EXPLANATION

To convert the measure of an angle in degrees to an equivalent measure in radians, we multiply the measure in degrees by \dfrac{\pi}{180}.

Evaluating this using a calculator, we get

30.2^\circ \cdot \dfrac{\pi}{180^\circ} \approx 0.527, rounded to 3 decimal places.

Therefore, the angle 30.2^\circ is equal to 0.5 radians rounded to one decimal place.

FLAG

Rounded to one decimal place, $99^\circ$ is equal to

a
b
c
d
e

Rounded to one decimal place, what is $48.6^\circ$ measured in radians?

a
$0.8$
b
$0.7$
c
$0.4$
d
$0.3$
e
$0.5$

The table below summarizes the values of some special angles in degrees and radians.



Degrees \:30^\circ\: \:45^\circ\: \:60^\circ\: \:90^\circ\: \:120^\circ\: \:135^\circ\: \:150^\circ\: \:180^\circ\:
Radians \dfrac{\pi}6 \dfrac{\pi}4 \dfrac{\pi}3 \dfrac{\pi}2 \dfrac{2\pi}3 \dfrac{3\pi}4 \dfrac{5\pi}6 \pi
Degrees \:210^\circ\: \:225^\circ\: \:240^\circ\: \:270^\circ\: \:300^\circ\: \:315^\circ\: \:330^\circ\: \:360^\circ\:
Radians \dfrac{7\pi}6 \dfrac{5\pi}4 \dfrac{4\pi}3 \dfrac{3\pi}2 \dfrac{5\pi}3 \dfrac{7\pi}4 \dfrac{11\pi}6 2\pi

Radians might seem a weird way of measuring angles now, but as you learn about trigonometry, you'll come to realize that they can be very useful and convenient.

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