Find Coterminal Angles

Find coterminal angles Ac to a given angle A.

What are coterminal angles?
If you graph angles x = 30 and y = - 330 in standard position, these angles will have the same terminal side. See figure below.

two coterminal angles

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

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 Ac may be given by
Ac = -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 Ac may be given by
Ac = -200 - 360 = -560

Example 2: Find a coterminal angle Ac to angle A = - 17 π / 3 such that Ac 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 Ac may be given by
Ac = - 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
Ac = - 17 π /3 + 6 π = π / 3

Example 3: Find a coterminal angle Ac to angle A = 35 π / 4 such that Ac 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
Ac = 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) Ac = 20 , b) Bc = 220
2. Find a positive coterminal angle smaller than 2 π to angles
a) Ac = 7 π / 6 , b) Bc = 7 π / 4


More references on angles.