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### A Glossary of Mathematical Terms - I

#### Icosahedron

A 20-sided solid;, each face is in the shape of a triangle.

#### Identity element

In operations (addition, multiplication, ...) the element that is neitral as far as the outcome is concerned.

If and only if.

#### Imaginary number

A complex number with no real part.

See fraction.

#### Improper subset

The null subset of a set or the whole set as a subset.

#### Integer

A member of the set {0, +1, -1, +2, -2, ... }.

#### Intercept

The point where a curve meets the x-axis (x-intercept) or the y-axis (y-intercept).

#### Interior angles

Usually referred to the angles of a polygon.

#### Intersection of sets

The set consisting of all elements that are common to two or more sets.

#### Interval

The set of all real (rational) numbers betwenn two fixed real (rational) numbers.

#### Isometry

A tarnsformation that keeps the same size and shape of figures.

#### Isosceles

A polygon with two sides equal in length. Usually referred to triangles and trapezoids.

first published on september 01 2003 - last updated on september 01 2003