# Differentiate yourself in trigonometry

**Calculus**Level 3

\[ y = \cos(x) \cdot \ln \left(\frac{\sin(x)+1}{\cos(x)} \right) \]

Find the value of the constant \( a \) if \( y + y'' = a \cdot \tan(x) \).

**Note**: By \( y'' \) we mean the *'second derivative'* of \( y \).