Respostas

2013-07-31T19:20:40-03:00
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}