Provet that? : $cos(-x) -= cosx$

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Answer 1 · 2016-10-25T23:50:54

This is a proof

We use the sum of angles formula:
$cos(A+-B) -= cosAcosB+-sinAsinB$

Put $A=0$ and $B=-x$ to get
$cos(0-x) -= cos0cosx-sin0sinx$

And we know that $sin0=0$ and $cos0=1$ so:
$cos(-x) -= (1)cosx - 0$
$:. cos(-x) -= cosx$ QED

Provet that? : cos(-x) -= cosxcos(-x) -= cosx