e (Euler's number)
An irrational number, approximately 2.71828, that is the base of natural logarithms and appears in continuous growth and decay.
Edge
A line segment where two faces of a solid figure meet.
Elapsed time
The amount of time that passes between a start time and an end time.
Element
One of the objects in a set, written with the symbol ∈.
Elimination method
Solving a system of equations by adding or subtracting the equations so that one variable cancels out.
Ellipse
An oval-shaped curve made of all points whose distances to two fixed points, the foci, add up to the same total.
Empirical rule
In a normal distribution, about 68% of data fall within 1 standard deviation of the mean, 95% within 2 and 99.7% within 3; also called the 68–95–99.7 rule.
Empty set
A set with no elements, written ∅ or { }.
End behavior
What happens to the y-values of a function as x moves far to the left or far to the right.
Endpoint
A point at the end of a line segment or ray.
Equality
The state of two quantities having the same value, shown with the equal sign.
Equation
A mathematical sentence stating that two expressions are equal, joined by an equal sign.
Equation of a line
An equation whose graph is a straight line, often written in slope-intercept form y = mx + b.
Equiangular
Having all angles equal. A rectangle is equiangular because all four angles are 90°.
Equidistant
At the same distance from two or more points or objects.
Equilateral triangle
A triangle with three equal sides. Each of its angles measures 60°.
Equivalent expressions
Expressions that have the same value for every value of the variable, such as 2(x + 3) and 2x + 6.
Equivalent fractions
Fractions that name the same amount but have different numerators and denominators, such as 1/2, 2/4 and 4/8.
Equivalent ratios
Ratios that express the same relationship, such as 2 : 3 and 4 : 6.
Estimate
To find an approximate answer, often by rounding, or the approximate value found this way.
Euler's formula (polyhedra)
For any convex polyhedron, vertices minus edges plus faces equals 2: V − E + F = 2.
Evaluate
To find the value of an expression by substituting numbers for the variables and computing.
Even function
A function with f(−x) = f(x) for every x, so its graph is symmetric about the y-axis.
Even number
A whole number that can be divided by 2 with no remainder. Even numbers end in 0, 2, 4, 6 or 8.
Event
An outcome or set of outcomes of a probability experiment, such as rolling an even number on a die.
Expanded form
A way of writing a number as the sum of the values of its digits, such as 347 = 300 + 40 + 7.
Expected value
The long-run average result of a random experiment, found by multiplying each outcome by its probability and adding.
Experiment
In probability, an action or process with results that can be observed, such as flipping a coin 10 times.
Experimental probability
The probability of an event based on the results of actual trials: the number of times it happened divided by the number of trials.
Explicit formula
A formula that gives any term of a sequence directly from its position n, such as aₙ = 3 + 4(n − 1).
Exponent
A small raised number that shows how many times the base is used as a factor.
Exponent rules
Rules for simplifying powers, such as xᵃ · xᵇ = xᵃ⁺ᵇ, (xᵃ)ᵇ = xᵃᵇ and x⁰ = 1.
Exponential decay
A decrease by the same percent in each time period, modeled by y = a·bˣ with 0 < b < 1.
Exponential function
A function of the form f(x) = a·bˣ, where the variable is in the exponent.
Exponential growth
An increase by the same percent in each time period, modeled by y = a·bˣ with b > 1.
Expression
A combination of numbers, variables and operations with no equal sign, such as 3x + 5y − 2.
Exterior angle
The angle formed between one side of a polygon and the extension of the side next to it.
Exterior angle theorem
An exterior angle of a triangle equals the sum of the two interior angles that are not next to it.
Extraneous solution
A value found while solving that does not work in the original equation, often after squaring both sides.
Extrapolation
Using a trend in data to predict values outside the range of the data collected.