$\"\"$

Observe the figure:

The measures of the triangle for angle $\"\"$ are $\"\"$ and $\"\"$ .

$\"\"$ (Defintion of cosine function)

$\"\"$ (Substitute $\"\"$ and $\"\"$ )

$\"\"$ (Divide: $\"\"$ )

$\"\"$ (Inverse function)

$\"\"$ (Use calculator)

$\"\"$ .

$\"\"$

$\"\"$ .