Question

How do you evaluate `arccos(-1/2)` without a calculator?

Answer 1

`(2pi)/3;(4pi)/3`

Explanation:

Use trig table for special arcs and unit circle -->
`cos x = -1/2`--> There are 2 arcs that have the same cos value -->
`x = (2pi)/3` and `x = (4pi)/3`
General answers:
`x = (2pi)/3 + 2kpi`
`x = (4pi)/3 + 2kpi`