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# C3 help needed watch

1. Question 13

This what I've done so far: = sin(a+b)/cos(a-b)

= sinAcosB + cosAsinA / cosAcosB - sinAsinB
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2. Try using the right hand side, making it into 1/(tan1/2(B-A)) and using the formula for tan(B-A) that you're given and expanding. Then see how you can work them to be the same.
3. Still can't do it
4. Oh, you made a mistake right off the bat. SinA + SinB is not the same as Sin(A+B), and same for the CosA - CosB.

This is a mistake made very often, try make sure not to do it
5. Use the sum to product formulae.
6. (Original post by electriic_ink)
Use the sum to product formulae.
Never heard of that before
7. (Original post by The Shanus)
Oh, you made a mistake right off the bat. SinA + SinB is not the same as Sin(A+B), and same for the CosA - CosB.

This is a mistake made very often, try make sure not to do it
So what can I do?
8. (Original post by Vorsah)
So what can I do?
There are loads of things you can do, but some will take ages. I would probably do it in a way that would take ages, and that would be what I said earlier. Leave the LHS (Left-Hand Side) how it is, and just get the RHS into Sin's and Cos's.
9. (Original post by Vorsah)
Never heard of that before
I'm pretty sure you will have formulae in your formula book that tell you how to rewrite things like sinA + sinB in the form of something like ksinPcosQ and similarly for the cos part of the formula.

These should make the problem a lot simpler for you!

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Updated: October 13, 2013
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