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# Trig Identities C2 Help - no idea how to do these questions, need help ASAP. watch

1. Questions 11-13, thanks for reading and I hope someone can help

2. (Original post by TheKevinFang)
Questions 11-13, thanks for reading and I hope someone can help

I'm really surprised no-one's helped you yet! There's so many good maths people knocking about.

Question 11 seems a bit vague to me, and I'm not confident with my answer but I just did this:
Let's just say theta = x for simplicity

sinx = p/3 and cosx = q/7
arcsin(p/3) = arccos(q/7)

For twelve, I suggest you follow a similar method to what I've done above. Rearrange the two equations to make them equal to k, then manipulate the two equations to find theta. Using theta you can find k.

For question 13, expand the left hand side of the two equations using the addition and double angle formulas respectively.
You can use the expansion of cos2x = 0.5 to find an exact value for either sin x or cos x depending on which you use, then sub that into the expansion of the other equation.
This is C3, by the way. Was that a typo or did you think these were C2 questions?
3. (Original post by TheKevinFang)
Questions 11-13, thanks for reading and I hope someone can help
The only other thing I could suggest for Q11 is to draw two suitable right angled triangles and get an equation from the fact that the two triangles will be similar and so the ratio of corrsponding sides will be equal (assumes is acute so not advisable, possibly a method for Q12 though)

Alternatively rearrange your two expressions in Q11 to get and and then apply the Pythagorean identity.

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Updated: October 31, 2015
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