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# Further Trig Problem watch

1. I got given a problem today in class as a quick finishing question just before the bell rang. I attempted it in class but couldn't finish it.

Hence show that

Now, my problem is I don't think it can be solved. I discussed with 2 of my mates who also agreed and testing values on a calculator also gave different results.

What I assume the question is supposed to look like is

My working so far for the question 1.

If anyone has any ideas what I am doing wrong, please let me know Ideally my tonight because I want to do this question. Also, I just started this topic today in lesson and finished it within two hours so I have't had much practice so any tips would be appreciated
...
You're right that that original question is incorrect.

Now you want to find the value of . So set and your LHS should just be an evaluation of some basic trig ratios.
3. (Original post by RDKGames)
You're right that that original question is incorrect.

Now you want to find the value of . So set and your LHS should just be an evaluation of some basic trig ratios.
Thank you. I was starting to feel like an idiot

So, I get the following working out. Hopefully this is right but just want a confirmation.

Hence,

Lastly, I know dumb question but how do you derive the identities

You mean ... Not sure why you used here.

Also, you were't being asked to derive the answer, simply use the *already given* fact that by simply plugging in into both sides. LHS is just an the RHS is just . Express the LHS in terms of ratios, simplify, job done. No identity proving required.

The identities are actually:

and are given the name 'double angle formulae', and then it's only natural to substitute into the first one to get , and then just replace with to arrive at the results you used.

In order to prove identities, you need to work with triangles and/or compound angle formulae.
5. (Original post by RDKGames)
x
Sorry my bad. I was still thinking about the first one which was wrong. Thanks for your help anyway. Makes a lot more sense now.

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