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# Core 3 Maths Question Watch

1. Hi I'm currently doing a question in the Jan 07 Core 3 OCR, and I'm kind of a bit confused about a question.
It goes:
Express 4cos? – sin? in the form Rcos(?+a), where R is greater than 0 and a is between 0 and 90 degrees.

For me its a very simple q- for R i get sq root 17, but for ? I get a different answer to the mark scheme, and I want to know why.

Usually I do cos-^1 or sin-^1 of the other number divided by R.
Ie Cos-^1 (1/sq.r.17) or sin^-1 (4/sq.r.17)
Normally I get the right answer- but I don't- i got 76 degrees, the answer is 14 degrees- so you obviously have to do cos-^1 of (4/sq.r17)

Please can anybody tell me why? I think its because on this one its a 4c. MINUS instead of plus sin? but im not too sure.
Thankyou!

can't recognise theta so ? means theta symbol

2. You've already worked out that

Now compare the two expressions.

Now do the same for the other part of the expression, and you'll be able to find the value of
3. (Original post by Rokaf0)
Usually I do cos-^1 or sin-^1 of the other number divided by R.
Ie Cos-^1 (1/sq.r.17) or sin^-1 (4/sq.r.17)
Since this wants you to write it as , you don't take the other 'number' (coefficient) but instead take the same one.

So it is actually and .

If the question said to rewrite it in the form then you would take the other number.

Spoiler:
Show

(This is because the addition formula of and

So for an equation in the form we'd obtain in the form , or in the form )
4. Thankyou! I understand it now

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