You would write something like this:
$game_troop.turn_count == 1 ? formula1 : formula2
This will make formula1 to be used if the turn count is 1, otherwise it uses formula2.
So, the left side (from the question mark) is the condition, the right side is the branch itself.
The method between the question mark and double colon happens when the condition is true, and the method to the right-most happens when the condition is false.
Also, $game_troop.turn_count returns an integer number, so you can directly implement it into your formulas as well.
For example:
a.atk * 0.2 * $game_troop.turn_count
This will make your damage highly dependent on the turn count. Could be useful for some skills, I guess.