Armor and Penetration(Damage calc)
Excellent post regarding to armor and penetration had already been published by LITTLE STAR HR. SEC. SCHOOL.
Mobile Legends Armor Calculation
https://littlestardimapur.com/mobile-legends-armor-calculation/
I would like to write points about armor and penetration based on his graph.
1, Damage is halved at defense 120
This graph's x-axis is defense value, y-axis is damage reduction rate.
I added yellow circles to his graph1.
We can see damage become 50% at defense 120.
His graph1's formula is below.
y = 100 * (120/(120+x))
Both of physical defense and magic defense apply this formula.
2, Diminishing marginal defense effect
This graph's x-axis is defense value, y-axis is marginal defense effect.
Marginal defense effect means how y changes with respect to x.
At x=50, y= 0.41.
At x=120, y=0.21.
It means that additional purchase defense equipment at defense=120 has only half worth of at defense=50.
Both of physical defense and magic defense apply this formula.
3, early game def and pen
In early game, defense and penetration effect is big because of damage formula.
Heroes have around 20 physical defense at level 01.
If you buy 10 physical penetration, you can deal around 8% more damage.
(when x=20, damage 85.7. x=10, damage 92.3. 92.3/85.7=108%.)
If you buy 20 physical defense, you can decrease around 12% taking damage.
(when x=20, damage 85.7. x=40, damage 75.0. 75.0/85.7=88%.)
These effect decrease in late game because marginal defense(penetration) effect decrease as you can see it in the graph above.
4, equipment effect and order to purchase
In early game, cheap defense equipment(e.g. 
And, cheap penetration equipment(e.g. 
On the other hand, in late game, defense value's worth decrease.
So equipment passive skill and synergy are important for final build set.
e.g. if enemy have much defense value, penetration with rate equipment 
If enemy have less defense item, penetration with value equipment
URL for reference
https://littlestardimapur.com/mobile-legends-armor-calculation/
https://chillplayer2018.wixsite.com/chillandplay/single-post/mlbb-damage-formula
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