In Methodus ad disquirendam maximam et minimam and in De tangentibus linearum curvarum, Fermat developed a method (adequality) for determining maxima, minima, and tangents to various curves that was equivalent to differential calculus.
In his works "Methodus ad disquirendam maximam et minimam" and "De tangentibus linearum curvarum," Fermat devised a method (called adequality) that mirrored the principles of differential calculus for finding maximums, minimums, and tangents on various curves.