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) $\frac { \sqrt { 3 } } { 2 }$

Question

Quizplus · Accepted Answer

The cosine of $ \frac { 2 \pi } { 3 } $ (or 120 degrees) is $- \frac { 1 } { 2 }$. This is because $ \frac { 2 \pi } { 3 } $ is in the second quadrant where cosine values are negative, and the reference angle $ \frac { \pi } { 3 } $ (or 60 degrees) has a cosine of $\frac { 1 } { 2 }$.

Find the Exact Value Without Using a Calculator cos⁡(2π3)\cos \left( \frac { 2 \pi } { 3 } \right)cos(32π​)

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