Matlab画不出波形,

Matlab画不出波形,
f(t)=sin(2*pi*sym('f0')*t)-alpha*sin(2*pi*sym('f0')*t)*[u*(t-sym('t1'))-u*(t-sym('t2'))];
sym('f0')=50,sym('t1')=0.04,sym('t2')=0.1,alpha=0.2;
plot(f(t),t);
grid on
这个是命令里的Error in ==> one at 1
f(t)=sin(2*pi*sym('f0')*t)-alpha*sin(2*pi*sym('f0')*t)*[u*(t-sym('t1'))-u*(t-sym('t2'))];