sum/difference/half angle formula

I do not understand what to do for these problems. I am very lost can someone show me how to do one of these.

And I copied and pasted my homework on here so I didnt mess up the instructions or problem(s).

Use a sum or difference formula or a half angle formula to determine the value of the trigonometric functions. Give exact answers. Do not use decimal numbers. The answer should be a fraction or an arithmetic expression. If the answer involves a square root it should be enter as sqrt; e.g. the square root of 2 should be written as sqrt(2);

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