GEOMETRY GLOSSARY
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Title  Description  

Identity  As opposed to a conditional statement that is sometimes true, an Identity will always be true. The multiplicative identity is 1; the additive identity is zero. 

IfandOnlyIf (Iff)  A statement that shows a condition both necessary and sufficient for the assertion. 

IfThen Statement  The classic biconditional statement is often phrased as an IfThen proposition. 

Impossibility  Despite what some "possibility thinkers" espouse, some things are mathematically impossible. For example, an exact real number cannot be simultaneously irrational and rational. 

Incenter  The center of a circle inscribed within a polygon. For a triangle, it is the point of concurrence of the angle bisectors. 

Incircle  A circle inscribed within a regular polygon (or any triangle) is an Incircle. In a regular polygon, the radius of the Incircle is the apothem. 

Inductive Logic  Inductive Logic is the logic of afterthefact, or a posteriori. It results from observation of transpired events. 

Infinite  In common language, not countable in any practical manner. In math, having no bounds or boundary. 

Infinite Geometric Progression  When a geometric progression has a common ratio less than one (technically, a common ratio whose absolute value is less than one), then the Infinite Geometric Progression will converge to a limit. 

Infinitesimal  Infinitely small is Infinitesimal, so tiny that it occupies no space. While in human terms anything really small (a molecule) is Infinitesimal, in math the term means approaching zero in size. 

Infinity  That without bound; limitless. 

Initial Side of an Angle  In standard position, the Initial Side of an Angle is the ray along the positive xaxis, from the origin. 

Inscribed Angle  An angle inside a circle with its vertex on the circle is an Inscribed Angle. 

Inscribed Circle  This term is the same as Incircle, a circle inscribed within a polygon. 

Interior  Interior means within or "inbetween."  
Interior Angle  Any angle inside a geometric entity, or between geometric lines, is considered an Interior Angle. 

Intersection  Where geometric entities cross, or where sets have common elements, is termed an Intersection. 

Interval  The space or region between two defined values is an Interval. 

Interval Notation  With brackets or parentheses, depending on whether endpoints are included in the set, Interval Notation expresses the solution set for an inequality. 

Invariant  Constant. Not changing. Static. That which does not vary. 

Inverse  Inverse carries a lot of meanings within the language of mathematics. 

Inverse Trigonometric Function  Given a number, this function returns the angle whose trig function is the given number. 

Inverse, Conditional  Given an initial ifthen statement, the negative of both the hypothesis and conclusion provides the Inverse to the original statement. 

Iota  The ninth letter of the Greek alphabet, Iota means a very small amount. 

Irrational Number  An Irrational Number cannot be expressed exactly as the ratio of two integers. Irrational Numbers, when expressed as decimals, never repeat or terminate. 

Isosceles Trapezoid  A trapezoid (quadrilateral with one pair of parallel sides) whose nonparallel sides are congruent is termed an Isosceles Trapezoid. 

Isosceles Triangle  A triangle with two congruent sides. 
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