The circle is a special case of the ellipse though historically it was sometimes called a fourth type. If you use the number other than 3 it fails to meet the condition criteria for a given equation. Quartic equation for degree four.
The first term has a power other than 2.
By Karl Wallulis The three methods most commonly used to solve systems of equation are substitution elimination and augmented matrices. The 3 kinds of linear equations are. Which key features relate directly to each form. Equations in quadratic form are equations with three terms.