Step 1:

\

The trigonometric identity is $\"\"$ .

\

Consider $\"\"$ .

\

Sum formula of sine function : $\"\"$ .

\

Then $\"\"$

\

$\"\"$

\

Take out common factor $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$ .

\

Solution :

\

The identity $\"\"$ is verified.