where is the error how to adjust it

\% Detine the constants below theta0 =0.1 g=9.8;L=0.5; \% Define the angular position, velocity, and acceleration functions below theta (t)= theta cos(sqrt(g/I)t); dtheta (t)=diff( theta); ddtheta(t) =diff( dtheta ) \% Plotting code figure; hold on; grid on; T=[0.05.0]; fplot (theta(t), T ) fplot(dtheta(t),T) fplot(ddtheta(t),T) legend ( { 'Angular position', 'Angular velocity', 'Angular acceleration' } ) Unrecognized function or variable 'theta'. Error in solution (line 8) theta(t)=thetacos(sqrt(g/1)t)