# Math Dictionary e

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**E**

**e :**The number "e" is one of the most important numbers in mathematics.

The first few digits are: 2.7182818284590452353602874713527 (and more ...)

It is often called Euler's number after Leonhard Euler.

**Eccentricity :**How much a "conic section" (a circle, ellipse, parabola or hyperbola) varies from being circular. Example: A circle has an eccentricity of zero.

**Edge of a polyhedron: :**A line segment that connects two consecutive vertices of a polyhedron.

**Element :**A member of a set.

**Ellipse:**A set of points 1 P in a plane, such that the sum of the distances from P to two fixed points 1 F and 2 F is a given constant k . Any plane section of a circular conical surface which is a closed curve.

**Endpoint:**A point at either the end of a line segment, or arc, or the initial point of a ray.

**Equal Chords :**Two chords which are equidistant from the center are called equal chords.

**Equiangular :**A polygon with all interior angles congruent.

**Equation :**An equation says that two things are the same, using mathematical symbols.

**Equidistant :**At the same distance.

**Equilateral polygon:**A polygon with all sides congruent.

**Equilateral triangle :**A triangle with three congruent sides.

**Equinox :**The time of the year when day and night are each 12 hours long and the Sun is at the midpoint of the sky.This happens twice a year:

* Spring equinox * Autumn (Fall) equinox

**Equivalence relation:**A relation that exhibits the reflexive, symmetric, and transitive properties.

**Equivalent fraction :**Fractions which have the same value, even though they may look different.

Example 1/2 and 2/4 are equivalent, because they are both "half".

**Estimation :**Finding a value that is close enough to the right answer, usually with some thought or calculation involved.

**Euclidean Geometry:**The study of geometry based on definitions undefined terms (point, line and plane) and the assumptions of Euclid (c.a. 330 B.C.)

**Euclidean Parallel Postulate:**Any assumption equivalent to the following statement: If l is any line and P is any point not on l , then there exists exactly one line through P that is parallel to l .

**Euler line:**For any given triangle, the line that contains the circumcenter, the centroid and the orthocenter.

**Even number:**Any integer that can be divided exactly by 2.

The last digit will be 0, 2, 4, 6 or 8

**Exponent :**The exponent of a number shows you how many times the number is to be used in a multiplication.

**Expression :**Numbers, symbols and operators (such as + and ×) grouped together that show the value of something.

**External segment of a secant :**If a secant is drawn to a circle from an external point, the portion of the secant that lies outside the circle.

**Exterior of a geometric figure:**The set of all points outside a geometric figure.

**Exterior angle of a polygon:**An angle formed by a side of a polygon and the extension of an adjacent side.

**Math dictionary e**

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