Draw a right angled triangle.
Let theta be an angle, 1 will be the opposite side, and 3 will be the hypotenuse.
Can you find the adjacent side through Pythagoras' Theorem?
Then, cos of theta is adj/hyp.
You should get the answer
Given that , how does one find the exact value of without using a calculator?
The answer is supposed to be , but I don't see how one reaches it. I know the standard sine/cosine angles ( ...) and their corresponding Cartesian coordinates on the unit circle; is the result supposed to be derived from these? If so, how?