Does it have something to do with reference angles. One more thing. [snip]
Yes. The third and fourth quadrant angles are and , the 'calculator answer') respectively.
Then and where n is an integer.
Originally Posted by TI-84
[snip]How did you know it was radians as the units I should use? [snip]
When workng with models involving trigonometric functions, the implied unit for the angle will always be radians. Note that the formulae for the derivative of sin x, cos x, tan x etc are for x measured in radians.