So say I have an item that gives me AP but also gives me 30 magic penetration. If I use one of my AP moves on someone that only has 20 magic resistance, is there extra damage done due to the +10 penetration?
Armor Penetration and Magic Penetration work in an odd way. For example, let's say you get a Brutalizer. It penetrates that 15 armor. So 15 armor is ignored from your opponent. However, if you get a Last Whisper. It ignores 40% of their armor, so if they have 100 armor, you ignore 40 armor as opposed to Brutalizer ignoring only 15. Straight armor penetration is good early game.