Question

cos A = .7986, find angle A

Answer 1

A = inverse cosine ( 0.7886 )

A = 0.64583 rad

and

A = - 0.64583 rad

Because cos ( - A ) = cos A

Since the cosine has a period of 2π:

A = 2 π + 0.64583

and

A = 2 π - 0.64583

A = ± 0.64583 + 2 π

Remark

Some denote the inverse cosine by

cos^-1 ( A )

and some by

arccos ( A )

Answer 2

I make typo.

Write solution as:

A = 2 π n + 0.64583

and

A = 2 π n - 0.64583

A = ± 0.64583 + 2 π n

where

n = ±1 , ± 2 , ± 3 ...