how is magic pen on jax

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except, you know. his W and his R passive do Magic damage and tend to make up 30-50% of his damage sometimes...

30-50% is impossible unless you build ap (which is not that viable anyway) w is often paired with his Q if you do so(at max rank both) 56% of the damage will be physical (Q 230dmg + W 180dmg = 410, 230/410 = ~56%( this number get higher the more bonus ad Jax gets)

If you don't pair it with your Q that mean you use it to reset your auto attack which mean 2 basic attack. let's say you are lvl 9 (the lvl which w is maxed out) jax has 85 ad ( so 85*2 (each hit)(170) + 180 = 350, 170/350 48%)

For his R, at lvl 6 jax has 75 ad, if every 3 hit he does 100 magic he still does more physical ( 75*3(physical) + 100magic = 325 dmg, 325/225 69% physical.

at lvl 16 jax has 111 ad, if every 3 hit he does 225 magic he still does more physical (111*3(physical)(333) + 225magic = 558 dmg, 333/5585 59% physical.

if you really build 0 ad has 0 ad runes Q and e as little as you can you will do about 50% magical which no one does end game jax has about 225 which is about what the magic damage of his R every third hit and he can do his w the 2 other hit let say at best , he w every third hit for 225 jax still does 3225(physical)(675) + 2225(magical)(450) = 1125, 675/1125 0.6 or 60%. so his magical reach 40% if he can W every third hit(which your cd wont allow) and he NEVER Q or E and doesn't have titanic hydra or deadman plate( which is a must have on jax), and always attack in 3 hits.

In my experience, a normal build for jax does betwenne 20-25% magical (looking at my damage in the after-match lobby) so no Magic pen on jax is bad only 25% of the damage will benefits from it

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First,No, you willl still do more physical damage i can give you the math if you want, stop not doing math and put out numbers. Second, i'd recommend you to focus the adc and the not armor stacking champion. you'll get better result.

splitpushing not teamfighting

It isn't a bad item on him, but there are better options.

If you still want another damage item after Hextech Gunblade and Trinity Force, I'd probably recommend Guinsoo's Rageblade, Maw of Malmortius or Frozen Mallet over Phantom Dancer.