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When deriving the equation for the superposed amplitude:

$$A^2=A_1^2+A_2^2+2A_1A_2 \cos(\phi_2-\phi_1)$$

From $$x_1(t)=A_1 \cos(\omega t+\phi_1)$$and $$x_2(t)=A_2 \cos(\omega t+\phi_2)$$

How do you determine which way the phase differences should be within the cos? Some examples I have seen say it is $\phi_1-\phi_2$ but it was shown as $\phi_2-\phi_1$ in my lecture notes.

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  • $\begingroup$ I think we use a modulus instead of brackets. This will probably solve your confusion! $\endgroup$ – Creepy Creature Oct 15 '19 at 19:09
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I guess it doesn´t really matter, since the cos(x) function is an even function, and thus cos(x) = cos (-x).

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