Talk:Magic resistance/@comment-205.154.159.114-20120112195533/@comment-70.169.50.209-20120426155147

And this is why i build arp for ad and map for ad. Negating the effective health of your opponent decreases the cost effectiveness of building arm/mr in the first place, aka making them have to choose health vs arm/mr. We will use armor penetration for my example.

Armor pen works by multiplying the targets armor by (100% - the percent of armor penetration you have)

Example 1. 20% armor penetration causes the target to take damage as if its armor was multiplied by 80%, so a target with 40 armor will be treated as though it had 32 for purposes of damage calculation (the target's actual armor value will not change, however). So lets look at what happens when they have 200 armor with the same armor penetration. Then they only have 160 armor for damage calculations and yet we didn't add anymore armor penetration.

With this example its plain to see that at a certain point you are actually losing effectiveness the higher armor you get against someone with lets say a Last whisper. While you may be getting more it will cost you more to get less than switching to health items.