In economics, when you plot cost and revenue on the Price-Quantity axis, the profit maximization condition is when marginal cost is equal to marginal revenue. This is a crucial notion to understand. Without it one can't effectively analyze profits. Does this make sense?
Yes, this is a correct notion. This comes from the theory of maxima and minima in differential calculus which says that the value of a function takes it maximum or minimum value where its first derivative is equal to ...
The theory of maxima and minima in differential calculus is cited.