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# C3 - Solomon G - Q2(b) - Help. watch

1. I can't seem to get the answer for this question - or understand the mark scheme.

(a) Use the identities for cos (A + B) and cos (A − B) to prove that
2 cos A cos B ≡ cos (A + B) + cos (A − B).
I did this easily

(b) Hence, or otherwise, find in terms of π the solutions of the equation
,
for x in the interval .

I opened up the RHS, turned sec into 1/cos and multiplied through by cos and then simplified down to:

now, I'm stuck. I'm sure I went wrong somewhere, but I have no clue where.
2. Think you might have made a mistake using the identity. Consider this

where i've used the identity for 2 cos A cos B you gave, defining

3. The question says hence, therefore this question relates to the identity on the previous question, you are expected to apply that identity to this question.

Hint: let A = x + pie/2, and B = x + pie/6
Think you might have made a mistake using the identity. Consider this

where i've used the identity for 2 cos A cos B you gave, defining

I actually ignored the identity given, because I had no clue where to use it! But now, it makes perfect sense. thank you!

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Updated: January 18, 2010
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