Question

How do you evaluate `Arc cos (4/5)`?

Answer 1

`x = 36^@87 + k360^@`

Explanation:

`cos x = 4/5` --> find arc x
Calculator gives:
cos x = 0.8 --> arc `x = 36^@87`
Unit circle gives another arc `x = - 36^@87` that has the same cos value.
Extended answers:
`x = +- 36^@87 + k360^@`