$\"\"$

The angle of rotation $\"\"$ can be defined as $\"\"$ .

The angle of rotation $\"\"$ can be defined as $\"\"$ , when $\"\"$ and $\"\"$ .

But it cannot be defined when $\"\"$ .

Because at $\"\"$ , $\"\"$ .

The angle of rotation $\"\"$ can be defined as $\"\"$ .

Apply reciprocal identity : $\"\"$ .

$\"\"$

Therefore, the cotangent terms are used to find the angle of rotation $\"\"$ .