弱まるvs弱る

what is the difference between 弱まる and 弱る? They are both intransitive and mean to be weaken.

How are they used differently?

intransitive from adjective stem + 「まる」has comparative meaning.
弱まる:become weaker than previous state
高まる:become higher than previous state
広まる:become wider than previous state

and focus on process.
intransitive from adjective stem + 「る」focus on just result.
弱る:become weak
太る:become fat
鈍る:become dull