Evaluate the Trigonometric Function Using Its Period as an Aid $$\cos \frac { 9 \pi } { 4 }$$

Multiple Choice

Evaluate the trigonometric function using its period as an aid. $$\cos \frac { 9 \pi } { 4 }$$

A) $\cos \frac { 9 \pi } { 4 } = - \frac { \sqrt { 2 } } { 2 }$
B) $\cos \frac { 9 \pi } { 4 } = - \sqrt { 2 }$
C) $\cos \frac { 9 \pi } { 4 } = \sqrt { 2 }$
D) $\cos \frac { 9 \pi } { 4 } = \frac { \sqrt { 2 } } { 2 }$
E) $\cos \frac { 9 \pi } { 4 } = \infty$