A cubic curve is an algebraic curve of curve order 3. An algebraic curve over a field 
is an equation 
, where 
is a polynomial in 
and 
with coefficients in 
, and the degree of 
is the maximum degree of each
of its terms (monomials).
Examples include the cissoid of Diocles, conchoid of de Sluze, folium of Descartes, Maclaurin trisectrix, Maltese cross curve, right strophoid, semicubical parabola, serpentine curve, Tschirnhausen cubic, and witch of Agnesi, as well as elliptic curves such as the Mordell curve and Ochoa curve.
Newton showed that all cubics can be generated by the projection of the five divergent cubic parabolas. Newton's classification of cubic curves appeared in the chapter "Curves" in Lexicon Technicum by John Harris published in London in 1710. Newton also classified all cubics into 72 types, missing six of them. In addition, he showed that any cubic can be obtained by a suitable projection of the elliptic curve
 |
(1)
|
where the projection is a birational transformation, and the general cubic can also be written as
 |
(2)
|
Newton's first class is equations of the form
 |
(3)
|
This is the hardest case and includes the serpentine curve as one of the subcases. The third class was
 |
(4)
|
which is called Newton's diverging parabolas. Newton's 66th curve was the trident of Newton. Newton's classification of cubics was criticized by Euler because it lacked generality. Plücker later gave a more detailed classification with 219 types.
The nine associated points theorem states that any cubic curve that passes through eight of the nine intersections of two given cubic curves automatically passes through the ninth (Evelyn et al. 1974, p. 15).
Pick a point 
,
and draw the tangent to the curve at 
. Call the point where this tangent intersects
the curve 
.
Draw another tangent and call the point of intersection with the curve 
. Every curve of third degree has the property that, with the
areas in the above labeled figure,
 |
(5)
|
(Honsberger 1991).
