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# Another C4 question

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1. Hi.
https://983c9f06eb1f75af6e83364e092b...20C4%20OCR.pdf
How is question 7(i) done.
I cant see how to get two equations.
Thanks
2. (Original post by SamuelN98)
Hi.
https://983c9f06eb1f75af6e83364e092b...20C4%20OCR.pdf
How is question 7(i) done.
I cant see how to get two equations.
Thanks
Expand your brackets to (in gonna use x to replace theta)
Asinx + Acosx+Bcosx-Bsinx=4sinx
So you have 2 equations here, one in terms of cosx and one in terms of sinx
Ainx - Bsinx=4sinx
Acosx+Bcosx=0

Therefore you get A-B=4, A+B=0
3. IF you expand it out you get .
4. You could also note that
when

Therefore

As such

You can then get:

Therefore

Thats just another way if you get stuck with the equations method (or expanding is a pain). Usually a longer way though
5. In the same vein as above, using immediately gives you and using gets you .
6. (Original post by Zacken)
In the same vein as above, using immediately gives you and using gets you .
How did you chose suitable values of x to substitute in?
7. (Original post by SamuelN98)
How did you chose suitable values of x to substitute in?
I wanted to choose values that made one of sine and cosine 0 in turn so that I could have A(one thing only) + B(one thing only) instead of A(sqrt(3)/2 + (1/2)) + B(blah) or the likes. So fit perfectly.

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