Are stacked one-handed damage perks additive or multiplicative?

Solution 1:

Multiplicatively. (Otherwise, it would take 5 ranks to increase dagger sneak attack damage from 15x to 16x.)

First off, this is the damage formula:

displayed damage = (base damage + item quality) * (1 + 0.4 * skill/100) *
                   (1 + perk effects) * (1 + item effects)

and for sneak attacks, most damage except that from poison or enchantments should be multiplied.

So assuming the no-ranks-in-Armsman, non-sneaking attack is x, then it breaks down like so for one-handed weapons:

+--------------+-----------+-----------+
|              | armsman 0 | armsman 5 |
+--------------+-----------+-----------+
| regular      |     x     |    2x     |
| sword sneak  |    6x     |   12x     |
| dagger sneak |   15x     |   30x     |
+--------------+-----------+-----------+