TRIGONOMETRY GLOSSARY
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Title  Description  

i, Square Root of 1  The smallcase i is reserved for the squareroot of a negative one; the square of i is 1. 

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. 

Imaginary Number  Imaginary Numbers exist, but we do not call them "real."  
Incenter  The center of a circle inscribed within a polygon. For a triangle, it is the point of concurrence of the angle bisectors. 

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

Infinite Series  Any series of terms whose progression has an unlimited (limitless) number of terms is an Infinite Series. 

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

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

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

Inverse Cosecant  Given a number, this function returns the angle whose cosecant is the given number. 

Inverse Cosine  Given a number, this function returns the angle whose cosine is the given number. 

Inverse Cotangent  Given a number, this function returns the angle whose cotangent is the given number. 

Inverse Secant  Given a number, this function returns the angle whose secant is the given number. 

Inverse Sine  Given a number, this function returns the angle whose sine is the given number. 

Inverse Tangent  Given a number, this function returns the angle whose tangent is the given number. 

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

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

Isosceles Triangle  A triangle with two congruent sides. 
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