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I'm currently learning trig identifies and on Khan academy, there is a graph demonstrating how sine and cosine addition identities are derived

enter image description here

I understand how each of the length is derived.

The author also mentioned

A similar diagram could show the angle difference identities. Can you draw one?

I can derive the sine and cosine angle difference identities algebraically.

Below is how I derive the sine angle difference identity. (by using the fact that cos(-b) = cos(b) and sin(-b) = -sin(b))

enter image description here

But I'm not sure how to derive the sine and cosine angle difference identities graphically. Could someone please provide me with a hint?

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    $\begingroup$ See my answer here. (It's more than a hint!) $\endgroup$ – Blue Sep 30 '18 at 5:09
  • $\begingroup$ very helpful indeed. Thanks :) $\endgroup$ – Thor Sep 30 '18 at 5:15

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