>>11390951Besides what
>>11391007 said, dx/dr is just a very small quantity of x divided by a very small of r, the arithmetic will be just x/r anyway.
People would be more aware of this if USAer universities didn't just go 'AYYY DUUDE LITERALY JUST DO POWER RULE CHIAN RULE INVERSE RULE SUBSTITUTION RULE MICROBRAIN RULE' instead of properly teaching the workings of it, but there is only so much time for a Calc 1 course
You could try and use the limit definition as well
