how to deduce cos rule, from sine rule??????

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- Oct 21st 2013, 07:47 PMsrirahulanTrig
how to deduce cos rule, from sine rule??????

- Oct 21st 2013, 08:05 PMProve ItRe: Trig
In short, you don't. To derive the Cosine Rule, you draw in the height of your triangle and apply Pythagoras' Theorem to the resulting right-angle triangle.

http://i22.photobucket.com/albums/b3...ps33b193fb.jpg

Or in the case of an obtuse-angle triangle...

Attachment 29538

We use the fact that and , then by Pythagoras we have

which upon simplification gives the same result.