Find the Exact Value Without Using a Calculator $\cos \left( \frac { 2 \pi } { 3 } \right)$

Multiple Choice

Find the exact value without using a calculator.
- $\cos \left( \frac { 2 \pi } { 3 } \right)$

A) undefined
B) $- \frac { 1 } { 2 }$
C) $- \frac { \sqrt { 3 } } { 2 }$
D) $\frac { \sqrt { 3 } } { 2 }$