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# Trig Identities

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1. Hi, I did a practise question for my tutor on trig Identities. The question was give the exact value for 2 sin 15 degrees cos 15 degrees.

I couldn't find a rule to apply to this so I just took the question literally and put the figures in the calculator, showing each step.

My tutor said that this was wrong and didn't give much explanation as to what I should do. I would be extremely grateful if someone could tell me how to go about this question.

2. (Original post by 153jam)
Hi, I did a practise question for my tutor on trig Identities. The question was give the exact value for 2 sin 15 degrees cos 15 degrees.

I couldn't find a rule to apply to this so I just took the question literally and put the figures in the calculator, showing each step.

My tutor said that this was wrong and didn't give much explanation as to what I should do. I would be extremely grateful if someone could tell me how to go about this question.

Note that sin(2x) = 2 sin x cos x.
Thus 2 sin 15 cos 15 = sin(2*15) = sin(30) = 1/2.
3. You could apply a sine rule (I think its called the double angle formula): 2sinxcosx=sin2x

so 2sin15cos15 = sin(2*15) which is sin30

I think you are expected to know the exact value of sin30 so maybe that's what he was after?

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