# Help checking compound angles question

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• February 22nd 2011, 07:37 AM
trigpoint
Help checking compound angles question
Hi
Forgive me if this is the incorrect forum - I consider compound angles to be trig, but maybe they're not!

I'm struggling to obtain the same answer as that given in a textbook. I'll give you my answer - I'd appreciate any help where I might have gone wrong:

Convert $-3 \, sin\,\omega t-6 \,cos\,\omega t$ to the form $R\,sin\,(\omega t\pm \alpha)$

I make this $6.708sin(\omega t- 4.2487)$

Is this in the 3rd quadrant as I think it is? The answer the book gives seems to imply the answer is in the second quadrant.
• February 22nd 2011, 09:37 AM
mestmoha
answer correct!
• February 22nd 2011, 09:50 AM
trigpoint
Quote:

Originally Posted by mestmoha
answer correct!

I think I figured out why they differ. For it to be in the 3rd quadrant, I assume the angle has to be *less* than pi, else it will fall in the 1st or 2nd quadrant (as it is a negative angle, and we're working back from zero).

I'm starting to think the book is right :)