# How do you evaluate Arc cos (4/5)?

Jun 12, 2017

$x = {36}^{\circ} 87 + k {360}^{\circ}$

#### Explanation:

$\cos x = \frac{4}{5}$ --> find arc x
Calculator gives:
cos x = 0.8 --> arc $x = {36}^{\circ} 87$
Unit circle gives another arc $x = - {36}^{\circ} 87$ that has the same cos value.
$x = \pm {36}^{\circ} 87 + k {360}^{\circ}$