Find the Exact Value -arccos(cos(pi))

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the second quadrant.

The exact value of ${\displaystyle \cos \left(0\right)}$ is ${\displaystyle 1}$.

Multiply ${\displaystyle -1}$ by ${\displaystyle 1}$.

The exact value of ${\displaystyle \arccos (-1)}$ is ${\displaystyle \pi }$.