If I trip with a weapon do I add the Weapon Focus bonus to the trip attack roll?

Question

Some special attacks such as Trip can be performed with weapons. Its text says:

Some weapons can be used to make trip attacks. In this case, you make a melee touch attack with the weapon instead of an unarmed melee touch attack, and you don’t provoke an attack of opportunity.

Supposing I am using a Flail (or other weapon which allows tripping with it) and I have Weapon Focus with the Flail, do I add the Weapon Focus bonus to the Trip check (i.e., the melee touch attack with the flail)?

P.s.: similar to this question, but I want an answer for D&D 3.5 instead of Pathfinder

Answer 1

Yes.

Weapon Focus provides the following benefit (emphasis mine):

You gain a +1 bonus on all attack rolls you make using the selected weapon.

As quoted in your question, when you use a weapon to trip, you make your trip attack with that weapon (emphasis again mine):

Some weapons can be used to make trip attacks. In this case, you make a melee touch attack with the weapon instead of an unarmed melee touch attack, and you don’t provoke an attack of opportunity.

Touch attacks are attacks, and they use attack rolls. From the SRD:

Some attacks disregard armor, including shields and natural armor. In these cases, the attacker makes a touch attack roll (either ranged or melee).

So, to sum up;

• When you trip with a weapon, you make a touch attack with the weapon.
• When you make a touch attack, it includes an attack roll.
• Weapon Focus provides a bonus on all attack rolls you make with the selected weapon.

Therefore, Weapon Focus provides its bonus on trip attacks made with the selected weapon.