Sorcery, XR (1) Fireball deals X damage divided evenly, rounded down, among any number of target creatures and/or players. As an additional cost to play Fireball, pay {1} for each target beyond the first.1
Illus. Mark Tedin
If split for multiple targets, each target can receive only one of the splits. You may not multiply target something. [bethmo 1994/06/01]
If the damage to creatures is redirected to the player (via Jade Monolith or other effect), the damage from the Fireball can all be prevented with one use of a Circle of Protection. The trick is that all damage from one use of a spell/ability can be prevented at once. [WotC Rules Team 1996/10/03]
The mana spent for extra targets is not part of the mana cost. It's an extra cost. [D'Angelo 2005/12/01]