Does inferno dragon stack multiplicatively with rengars passive?

I was playing a game as olaf and had 280 armor and 3600 health and he burst me 100-0 with the following combo: AA>Q>AA>E>AA, his items were: dusk blade, ghostblade, trinity, Infinity edge, Lord Dominiks regards, and the jungle item red enchant. They had 3 inferno dragons, they did not have baron buff or elder dragon buff I am unsure how many crits he hit me with but he only had 20% crit chance with the IE. Kha'zix was not in the game.

What im most curious about is if rengar's passive damage stacks multiplicatively with inferno dragon buff, which would provide rengar with 61.2% bonus AD, i guess that doesnt make that much of a difference from 54% though.