I'm not sure that can be seen well but the error is 

Error, invalid input: First_Principle expects its 1st argument, f, to be of type appliable, but received tan(x)

and the result for sin^2(x) is 2*(sin(x))(x)*(D(sin(x)))(x)

@Carl Love Hi, I tried this 



but I get the incorrect sin result and the error?