How do you prove $cos 2 A = 2 {cos}^{2} A - 1$ $cos2A = 2cos^2 A - 1$ ?

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Answer 1 · 2015-04-20T19:43:03

Probably it is not very much as a "proof" but...
if $A=30°$
$2A=60°$
so:
$cos(60°)=1/2$
$2cos^2(30°)-1=2(sqrt(3)/2)^2-1=3/2-1=1/2$

How do you prove cos2A = 2cos^2 A - 1cos2A=2cos2A−1cos2A = 2cos^2 A - 1?