What is $cos(2arccos0.25)$ ?

Mar 12, 2017

$cos(2arccos0.25)=-0.875$

Let $arccos0.25=x$ , then $cosx=0.25$

Hence, $cos(2arccos0.25)$

$=cos(2x)$

$=2cos^2x-1$

$=2xx(0.25)^2-1$

$=2xx0.0625-1$

$=0.125-1$

$=-0.875$

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