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# Wave Function - Can someone please help me out? Watch

1. I understand the method of how to express something in the wave form however there is one small stage I'm having difficulties getting right and I don't understand what it is I actually do.

When I have to expand the likes of kcos(x-a), how do I do it? Where does the sin come from? I can do the rest but I keep making mistakes here with the positive and negatives. Can someone just tell me exactly what is being done when this wave form is expanded, what are you actually doing? I think if I can see how to expand it step-by-step will stop making mistakes.

Just so you know what I mean:

When I expand kcos(x-a) how do I know it's not:

1. k(cosxcosa - sinxsina) or
2. k(cosxsina - cosasinx) or
3. k(cosxcosa + sinxsina) or
4. k(cosxsina + cosasinx)

how do I expand it correctly? What am I actually multiplying together! Please help!! Thanks
2. It is on the formula sheet so you don't need to memorise them

But for this case it is, k(cosxcosa + sinxsina)
3. You are over-thinking this, it's just the double-angle formulae with a k stuck on the side.

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Updated: April 11, 2013
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