ts=0;te=5;dt=0.001;sys=tf([1,3],[1,4,4]);t=ts:dt:te;f=exp(-t).*Heaviside(t);y=lsim(sys,f,t);plot(t,y);xlabel('time(sec)');ylabel('y(t)');??? Error using ==> rfinputsInput data U contains Inf or NaN.哪里错了?????