Answer:
Critical points of an equation are the zeroes of its first derivative. Since you have already provided me with the derivative (thanks for saving me the effort :) ), I am just going to start the process.
tan (x) + x(sec^2(x)) = 0
Solving this equation (using a graphing calculator set to radians), it is only true when x = 0.
So, 0 is a critical point of this function.
Hope this helped!
