I wanted a midly rigorous proof to make the chain rule more intuitive to me, and this is what I found: http://web.mit.edu/wwmath/calculus/differentiation/chain-proof.html
However, I am not sure how how going from step two to three on this part of the proof is justified: http://web.mit.edu/wwmath/calculus/differentiation/chaineq/chaineq31.gif
High school sophomore in calc ab
Thanks for any help!! ☺