Question

 cos$5 \frac \pi 4$

Answer 1

Calculando, temos que:

$\dfrac{5\pi}{4}=225^{\circ}$

E:

$360^{\circ}-225^{\circ}=45^{\circ}$

Então $\cos225^{\circ}=-\cos45^{\circ}$:

$\cos225^{\circ}=-\cos45^{\circ}$

$\cos225^{\circ}=-\dfrac{\sqrt{2}}{2}$