This item COMPLETELY negates Infinity Edge

I do not see a joke in this. Care to explain it for me?

The math doesn't work that way, though.

Infinity Edge increases your critical strike multiplier from 2.0 to 2.5, which is 25% more damage.

In order to counteract a 25% damage increase, you need to get 20% damage reduction, because 1.25 times 0.8 = 1. If Randuin's stacks additively (that is, it provides 10% reduction with one and 20% with two), then you only need two randuin's to negate IE's passive. If it's multiplicative (randuin's #1 reduces it by 10%, then randuin's #2 reduces it by 10%) then you don't quite negate IE's passive (two randuin's would be 19% net reduction) but it's close enough as to make no practical difference.

Amusingly, if you get six of them, you could actually end up taking less critical strike damage than normal damage from a foe without IE. Not sure if that'd be viable on anything except maybe Rammus, but it'd be funny at the very least.

WITHOUT RUNES AND MASTERIES Normal Crit; 200% damage IE Crit: 250% damage

IE Crit reduced by randuin: 250%*90%*90%*90%*90%*90%=147.6225% damage Normal Crit reduced by randuin: 200%*90%*90%*90%*90%*90%=118.098% damage

I'm starting to see why people stack Lethality now...