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# C3 Trigonometry Exam Question help needed! Watch

1. Hey, I've been working through past papers as a part of my revision and I am really stuck on this question (it's from Edexcel June 2006):

8.a) Given that cosA=3/4, where 270degrees<A<360degrees, find the exact value of sin2A.

I was wondering if you had to use the identity sin2A=2sinAcosA?
2. Yes. That was my initial thought as well.

EDIT: Just remember to use the correct angle for A (i.e the angle within your given range).
3. Not sure if my method is the most concise but use s^2 = 1 - c^2, then square root and sub in. Remember, 270<A<360! Are your square roots negative or positive?

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4. Also you might like to use
5. (Original post by Mr M)
Also you might like to use
You know in questions like this, when they say "Find the exact value of sin(2A)", what should your final answer be? Because even in my Differential Geometry module, I get questions sometimes asking me to workout the cosine of two surfaces/ whatever and I don't know how to leave the final answer.

Should it be cos(A) = BLAH (or in this case sin(2A) = BLAH) do I actually need to work out what A would be?
6. (Original post by claret_n_blue)
You know in questions like this, when they say "Find the exact value of sin(2A)", what should your final answer be? Because even in my Differential Geometry module, I get questions sometimes asking me to workout the cosine of two surfaces/ whatever and I don't know how to leave the final answer.

Should it be cos(A) = BLAH (or in this case sin(2A) = BLAH) do I actually need to work out what A would be?
No you shouldn't find out A (although there is nothing wrong with doing so at the end just to check the answer has the correct magnitude and sign).

In your DG question they just want the value of the cosine not the angle itself.

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