MATLAB fplot of symbolic function

Question

Newbie Matlab question here. I'm sure I am missing something here but I can't seem to figure out why my fplot comes back empty, could someone point me in the right direction. I am attempting to plot the derivative of 2 functions.

syms x(t)%creates symbolic function x of t

alpha=atan(8/x);    % alpha relationship to x
beta=acos(sqrt(x^2+64)/40); % Beta relationship to x
phi=(pi-(alpha+beta)); % phi relationship to x
theta=(pi/2-phi); % theta relationship to x
Dtheta=diff(theta,1); % angular velocity of theta
pretty (Dtheta) % Prints angular velocity of theta in a more readable way
Dphi=diff(phi,1); % angular velocity of phi
pretty (Dphi) % Prints angular velocity of phi in a more readable way

fplot(Dtheta)
fplot(Dphi)

Sorry if this is a lame question!

Answer 1

Change the first line to :

syms x

Because you want to define functions of x not of x(t) .