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I'd like it explained through the unit circle as I find trig identities easier to understand in this manner. Any help would be appreciated, thanks!

EDIT: I know you have to apply the identity $\sin(x)=\cos(90-x)$, but I'm wondering how i'd visualise all this on the unit circle??

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This isn't on the unit circle, but I like it: math.stackexchange.com/a/1342/409 –  Blue Mar 3 '13 at 16:00
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4 Answers

up vote 3 down vote accepted

This is the proof of $\cos(a+b)=\cos a \, \cos b - \sin a \, \sin b$.

See also this proof.

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OOPS! I didnt see the link!, i'll check it now! –  Assad Mar 3 '13 at 15:28
    
This is for cosine, what would happen for sine? how would the triangles be positioned? I'm just confused how its derived.. This was the kind of thing i was looking for, thanks! –  Assad Mar 3 '13 at 15:39
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Using complex numbers and exponential form perhaps help (at least algebraically) to digest these trigonometric addition formulas:

All we have to know is $\cos a+i\cdot\sin a=e^{ai}$ for any $a\in\Bbb R$, and that $i^2=-1$, and that $e^{x+y}=e^x\cdot e^y$ for any $x,y\in\Bbb C$. Then calculate both sides of $e^{(a+b)i}=e^{ai}e^{bi}$.

If you prefer, instead, you can use the matrices of rotation: $$R_a:=\pmatrix{\cos a&-\sin a\\ \sin a &\cos a}$$ and use matrix multiplication to verify the identities, knowing that $$R_{a+b}=R_a\cdot R_b \ .$$

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I'm afraid I haven't learned some of the concepts mentioned in your answer, thank you for this however. –  Assad Mar 3 '13 at 15:31
    
And how do you know all those properties about complex numbers? –  André Caldas Mar 3 '13 at 16:03
    
Any proofs I know of for Euler's identity require the knowledge that $\frac{d}{dx}\sin x = \cos x$ and $\frac{d}{dx} \cos x = - \sin x$. The limit involved in these derivatives requires knowledge of the trig subtraction formulas, which means that the proof using this identity (as far as I can tell) is circular. –  Omnomnomnom Jan 14 at 22:49
    
The rotation matrix proof, however, isn't a bad idea. I have a hunch that it is essentially the same as the usual geometric proof. –  Omnomnomnom Jan 14 at 22:52
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This video will clear matters beautifully. Make sure to watch it, and then the next.

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The explanation given by eepsmedia here is what you are looking for. Although his argument gives the cosine addition formula, and only in the case when $\alpha+\beta < \pi/2$, you should be able to use the same methods to obtain the sine angle addition formula.

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