# How do you find the exact values cos (pi/4) using the special triangles?

$\cos \left(\frac{\pi}{4}\right) = \frac{1}{\sqrt{2}}$
The "special triangle" corresponding to $\frac{\pi}{4}$ is an isosceles triangle with equal adjacent and opposite sides (and a hypotenuse that can be calculated using the Pythagorean Theorem).
$\textcolor{w h i t e}{\text{XXXX}}$$\cos = \left(\text{adjacent")/("hypontenuse}\right)$