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 after my combat phase i played Rakdos, Lord of Riots . will his ability lower the cost of the creatures i cast this turn after i play him? or he will not affect their cost this turn since he wasnt on the field when i dealt the damage?

Rakdos doesn't need to have been on the battlefield when the life was lost in order to reduce the costs of your creature spells.

It will apply when casting creatures the turn it enters the battlefield.  The ability checks how much life was lost in the current turn, not how much life was lost while the ability existed, so any life lost by the opponent during combat will be applied to cost reduction that turn.