Coterminal angles point in the same exact direction.
In degree mode, you can add on 360 degrees to get a coterminal angle.
Example: 90 degrees is coterminal to 450 degrees because 90+360 = 450. Both angles 90 and 450 point directly north.
You can also subtract 360 from the angle to get a coterminal counterpart.
Example: Given the angle 700, it is coterminal to 340 since 700-360 = 340
This process of adding or subtracting 360 can be done as many times as you like.
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For radian mode, you'll add or subtract 2pi radians to get a coterminal angle. This is because 2pi radians = 360 degrees by definition.
Examples:
- pi is coterminal to 3pi because pi+2pi = 3pi, both point directly west
- pi/4 is coterminal to 9pi/4 because pi/4 + 2pi = pi/4 + 8pi/4 = 9pi/4
- 21pi/4 is coterminal to 13pi/4 because 21pi/4 - 2pi = 21pi/4 - 8pi/4 = 13pi/4