1. anonymous

2. Michele_Laino

we have this drawing: |dw:1438159678788:dw|

3. Michele_Laino

as we can see, we can write: $\begin{gathered} \cos \alpha > 0 \hfill \\ \sin \beta < 0 \hfill \\ \end{gathered}$

4. Michele_Laino

hint: therefore, we can write: $\Large \begin{gathered} \cos \alpha = \sqrt {1 - {{\left( {\sin \alpha } \right)}^2}} = ... \hfill \\ \sin \beta = - \sqrt {1 - {{\left( {\cos \beta } \right)}^2}} = ... \hfill \\ \end{gathered}$

5. Michele_Laino

furthermore, you have to use this identity: $\Large \sin \left( {\alpha + \beta } \right) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$