Is it true that when two functions intersect at one point (A), then their derivatives must be equal at A?
No! They might simply cross over at that point.
Ohh right! Just tested it with some functions, yes it doesn't always work.
But I noticed it works if and only if the functions are tangents to each other at that point ?
"But I noticed it works if and only if the functions are tangents to each other at that point ? "
Yes.
