Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Find the value of .
The special angles: , , , , ; the cosines are the same values in the opposite order. , , , and is not defined.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The squares. , , .
Step 2. Put them in. .
Answer: .
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