Find Coterminal Angles

Find coterminal angles A c to a given angle A.

What are coterminal angles?

If you graph angles α = 30° and β = - 330° in standard position, these angles will have the same terminal side and are therefore called coterminal angles. See figure below.

Coterminal angles

Coterminal angles A c to angle A may be obtained by adding or subtracting k × 360 degrees or k × (2 π). Hence
A c = A + k × 360°
if A is given in degrees.
or
A c = A + k ×(2 π)
if A is given in radians.
where k is any negative or positive integer.


Examples

Example 1

Find a positive and a negative coterminal angles to angle A = -200°

Solution to example 1:
There is an infinite number of possible answers to the above question since k in the formula for coterminal angles is any positive or negative integer.
A positive coterminal angle to angle A may be obtained by adding 360°, 2(360)° = 720° (or any other positive angle multiple of 360°). A positive coterminal angle A c may be given by
A c = -200° + 360° = 160°
A negative coterminal angle to angle A may be obtained by adding -360°, -2(360)° = -720° (or any other negative angle multiple of 360°). A negative coterminal angle A c may be given by
A c = -200° - 360° = -560°


Example 2

Find a coterminal angle A c to angle A = - 17 π / 3 such that A c is greater than or equal to 0 and smaller than 2 π

Solution to example 2:
A positive coterminal angle to angle A may be obtained by adding 2 π, 2(2 π) = 4 π (or any other positive angle multiple of 2 π). A positive coterminal angle A c may be given by
A c = - 17 π / 3 + 2 π = -11 π / 3
As you can see adding 2*π is not enough to obtain a positive coterminal angle and we need to add a larger angle but what is the size of the angle to add?. We need to write our negative angle in the form - n (2 π) - x, where n is positive integer and x is a positive angle such that x < 2 π.
- 17 π /3 = - 12 π / 3 - 5 π / 3 = - 2 (2 π) - 5 π / 3
From the above we can deduce that to make our angle positive, we need to add 3(2*π) = 6 π
A c = - 17 π /3 + 6 π = π / 3


Example 3

Find a coterminal angle A c to angle A = 35 π / 4 such that A c is greater than or equal to 0 and smaller than 2 π

Solution to example 3:
We will use a similar method to that used in example 2 above: First rewrite angle A in the form n(2π) + x so that we can "see" what angle to add.
A = 35 π / 4 = 32 π / 4 + 3 π / 4 = 4(2 π) + 3 π /4
From the above we can deduce that to make our angle smaller than 2 π we need to add - 4(2π) = - 8 π to angle A
A c = 35 π / 4 - 8 π = 3 π /4


Exercises

(see solutions below)
1. Find a positive coterminal angle smaller than 360° to angles
a) A = -700° , b) B = 940°
2. Find a positive coterminal angle smaller than 2 π to angles
a) A = - 29 π / 6 , b) B = 47 π / 4

Solutions to Above Exercises:
1.
a) A c = 20° , b) Bc = 220°
2. Find a positive coterminal angle smaller than 2 π to angles
a) A c = 7 π / 6 , b) Bc = 7 π / 4


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