I got it. One can check the sign without displaying the expanded expression. All one need is to use the command

assume(alpha_6>0, mu_1>0,.....) and then with the command signum(SC3)

Thanks you guys for your contribution..

Regards,

Plus the comment by Adri, you need also to define properly things....

f(0):=0.9..............

a:=5/9;

f(t):= a*...........

with(plots): plot(f(t), t=0..20);

And that's it.