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During our game, the fighter PC rolled a critical hit. After he made the critical hit, he wanted to use a superiority die to trip the enemy. Based on the below rules, is the superiority die also doubled? I'm assuming yes based on the last statement in the Critical Hits section, but I wanted to confirm.
Trip Attack. When you hit a creature with a weapon attack, you can expend one superiority die to attempt to knock the target down. You add the superiority die to the attack’s damage roll, and if the target is Large or smaller, it must make a Strength saving throw. On a failed save, you knock the target prone.
Critical Hits When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal.... If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well