# Trigonometry - Transforming of trigonometric expressions to sum

1. $sin\,\alpha.sin\,\beta={cos(\alpha-\beta)-cos(\alpha+\beta) \over 2}$

2. $cos\,\alpha.cos\,\beta={cos(\alpha-\beta)+cos(\alpha+\beta) \over 2}$

3. $sin\,\alpha.cos\,\beta={sin(\alpha-\beta)+sin(\alpha+\beta) \over 2}$

4. $tan \,\alpha.tan\, \beta={tan \,\alpha+tan \,\beta \over cot\, \alpha+ cot\, \beta }$

5. $cot \,\alpha.cot\, \beta={cot\, \alpha+ cot\, \beta \over tan \,\alpha+tan \,\beta }$

6. $tan \,\alpha.cot\, \beta={tan\, \alpha+ cot\, \beta \over cot \,\alpha+tan \,\beta }$