I am rather new to the whole idea of complex conjugates and especially operators. I was trying to understand the solution to a problem I was doing, but the math is confusing me rather than the physics. In the last row of calculations, why does the sin change to a cos, and the d/dx change to what it is. I recognize that n(pi)/L is the first derivative of the inside function for the trig functions, but don't understand how it got there. Thank you for any help and sorry in advance if I posted this in the wrong section, I'm not positive what section this would fall under.