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Conversion between Degrees and Radians

Convert between radians and degrees

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Introduction to the Unit Circle and Radian Measure

An odd-shaped house in Asia is built at a 135 angle. How many radians is this angle equal to?

Guidance

The unit circle is the circle centered at the origin with radius equal to one unit. This means that the distance from the origin to any point on the circle is equal to one unit.

Using the unit circle, we can define another unit of measure for angles, radians. Radian measure is based upon the circumference of the unit circle. The circumference of the unit circle is 2π ( 2πr , where r=1 ). So a full revolution, or 360 , is equal to 2π radians. Half a rotation, or 180 is equal to π radians.

One radian is equal to the measure of θ , the rotation required for the arc length intercepted by the angle to be equal to the radius of the circle. In other words the arc length is 1 unit for θ=1 radian.

We can use the equality, π=180 to convert from degrees to radians and vice versa.

To convert from degrees to radians, multiply by π180 .

To convert from radians to degrees, multiply by 180π .

Example A

a. Convert 250 to radians.

b. Convert 3π to degrees.

Solution:

a. To convert from degrees to radians, multiply by π180 . So, 250π180=25π18 .

b. To convert from radians to degrees, multiply by 180π . So, 3π×180π=3×180=540 .

Example B

Find two angles, one positive and one negative, coterminal to 5π3 and find its reference angle, in radians.

Solution: Since we are working in radians now we will add/subtract multiple of 2π instead of 360 . Before we can add, we must get a common denominator of 3 as shown below.

5π3+2π=5π3+6π3=11π3and5π32π=5π36π3=π3

Now, to find the reference angle, first determine in which quadrant 5π3 lies. If we think of the measures of the angles on the axes in terms of π and more specifically, in terms of π3 , this task becomes a little easier.

Consider π is equal to 3π3 and 2π is equal to 6π3 as shown in the diagram. Now we can see that the terminal side of 5π3 lies in the fourth quadrant and thus the reference angle will be:

6π35π3=π3

Example C

Find two angles coterminal to 7π6 , one positive and one negative, and find its reference angle, in radians.

Solution: This time we will add multiples of 2π with a common denominator of 6, or 2π1×66=12π6 . For the positive angle, we add to get 7π6+12π6=19π6 . For the negative angle, we subtract to get 7π612π6=5π6 .

In this case π is equal to 6π6 and 2π is equal to 12π6 as shown in the diagram. Now we can see that the terminal side of 7π6 lies in the third quadrant and thus the reference angle will be:

7π66π6=π6

Concept Problem Revisit

To convert from degrees to radians, multiply by π180 . So, 135π180=3π4 .

Guided Practice

1. Convert the following angle measures from degrees to radians.

a. 45

b. 120

c. 330

2. Convert the following angle measures from radians to degrees.

a. 5π6

b. 13π4

c. 5π2

3. Find two coterminal angles to 11π4 , one positive and one negative, and its reference angle.

Answers

1. a. 45×π180=π4

b. 120×π180=2π3

c. 330×π180=11π6

2. a. 5π6×180π=150

b. 13π4×180π=585

c. 5π2×180π=450

3. There are many possible coterminal angles, here are some possibilities:

positive coterminal angle: 11π4+8π4=19π4 or 11π48π4=3π4 ,

negative coterminal angle: 11π416π4=5π4 or 11π424π4=13π4

Using the coterminal angle, 3π4 , which is π4 from 4π4 . So the terminal side lies in the second quadrant and the reference angle is π4 .

Explore More

For problems 1-5, convert the angle from degrees to radians. Leave answers in terms of π .

  1. 135
  2. 240
  3. 330
  4. 450
  5. 315

For problems 6-10, convert the angle measure from radians to degrees.

  1. 7π3
  2. 13π6
  3. 9π2
  4. 3π4
  5. 5π6

For problems 11-15, find two coterminal angles (one positive, one negative) and the reference angle for each angle in radians.

  1. 8π3
  2. 11π4
  3. π6
  4. 4π3
  5. 17π6

Vocabulary

subtended arc

subtended arc

A subtended arc is the part of the circle in between the two rays that make the central angle.

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