Guild Wars 2

as a ranger, if I have steady focus (10% dmg while full endurance), hunter’s tactics (10%dmg while flanking), and runes of the ranger (+7% dmg with companion) how do those all add up? how much extra damage would I actually be doing? does it stack multiplicatively?

According to the wiki :
Damage multipliers are combined multiplicatively. For example the Superior Sigil of Force (5%), Superior Sigil of the Night (10%), a 10% trait, a 20% trait and the (6) bonus of the Superior Rune of the Scholar (10%) combine to roughly 68% more damage (1.05 * 1.1 * 1.1 * 1.2 * 1.1 ? 1.677), not only 55% (the summation). Consequently, combining a lot of multipliers is exponentially effective (“the multipliers increase/affect each other”).
http://wiki.guildwars2.com/wiki/Damage_calculation

If they were additive we wouldn’t call them multipliers, just sayin’

oxtred.7658:

If they were additive we wouldn’t call them multipliers, just sayin’

They would still be multipliers. You would just be multiplying the base damage by the sum of the multipliers instead.

If it was me I’d call that an additioner. Or a summer but that’s already taken. Maybe a pluser.