@dharr @Carl Love

Thanks for pointing out "simplify" just simplify LHS and RHS separately.

However, in my case, it is not an algebraic equations, which means f(x) contains derivatives. The "solve" can not give what I want.

For example, I arrive at:

It may not be analytically solvable, so I want to clean it a little bit for further consideration. 

I am a new user for Maple from Mathematica. I notice in Mathematica we can do this:

Simplify[f[x]*a=0, a>0], which gives: f[x].

I'm thinking if maple can do the similar job.

Thanks!