Theorems, equations, and techniques named after people other than their creators.
(from Chalk Up Another One by S. Harris)
This theorem relates the three sides of a right triangle:
The sum of the squares of the legs equals the square of the hypotenuse.
This theorem was known in at least four ancient civilizations before
Pythagoras's time: Babylonian, Indian, Greek, and Chinese.
This theorem relates the number of faces, edges, and vertices in a
polyhedral:
F+V-E=2
Where F is number of faces, V is the number of vertices,
and E is the number of edges.
The theorem appears in some of René Descartes unpublished papers
(ca. 1635). Euler announced his theorem in 1752.
When both the numerator and denominator of a fraction both approach zero, you can use L'Hôpital's rule to determine the value of the fraction. The value of the fraction is equal the derivitive of the numerator devided by the derivitive of the denominator. This rule was discovered by Johann Bernoulli, who worked for L'Hôpital.
A determinant is a function that assigns a number (a scalar) when given a square table of numbers (a square matrix). They are used in finding areas, volumes, and solving systems of simultaneous equations. Determinants appear in a letter from Leibniz to L'Hôpital (1693), but the Japanese mathematician Takakazu Seki introduced the work earlier (1683).
I should note my two sources give different dates for Leibniz's letter. Source (1) puts the letter at 1693, source (2) puts the letter at 1683 but does not give a date for Seki's work.
In 1545 Jerome Cardan printed Niccolò Tartaglia's formula for finding the roots of a cubic equation. Cardan wormed the formula from Tartaglia, who swore he would never tell.
Diophantine equations are indeterminate linear equations where you are only interested in integer solutions. Several Hindu mathematicians were interested in these equations. Diophantus of Alexandria (ca. 250) was not interested in Diophantine equations.
Cramer's rule is a method of using determinants for solving systems of simultaneous equations. Cramer published his method in 1750, but the method was published in a 1748 posthumous book by Colin Maclaurin.