# Online Math Dictionary: I

Easy to understand math definitions for K-Algebra mathematics
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 icosahedron icosidodecahedron identityadditive identity identitymultiplicative identity imaginary number improper fraction inequality infinite infinity inscribed integer intersection intersection of sets inverseadditive inverse inversemultiplicative inverse irrational number isosceles triangle isosceles trapezoid

 IcosahedronAn icosahedron is a polyhedron made up of twentyequilateral triangles.  It is one of the five Platonic Solids.     20 faces: equilateral triangles     12 vertices     30 edges     Dihedral angle: about 138.19 degrees For more info about Platonic solids, check out myPlatonic solids gallery.  IcosidodecahedronAn icosidodecahedron is a polyhedron - one of the thirteen semi-regular Platonic solids.  It's created by either cutting (truncating) the dodecahedron one half of the way into each side or cutting (truncating) the icosahedron one half of the way into each side.     32 total faces: 20 equilateral triangles and                                  12 regular pentagons      30 vertices: 2 triangles and 2 pentagons     60 edges     Dihedral angle: about 142.623 degrees For more info about Platonic solids, check out myPlatonic solids gallery. Identity - Additive IdentityIn basic arithmetic, the additive identity is the number 0...  Because you can ADD 0 to any number and that number keeps its identity (it stays the same).Example:   5 + 0 = 5 Identity - Multiplicative IdentityIn basic arithmetic, the multiplicative identity is the number 1...  Because you can MULTIPLY 1 to any number and that number keeps its identity (it stays the same).Example:   3 x 1 = 3 Imaginary Number
The regular numbers you usually work with are from the Real Number System.  But, there is another kind of number - an imaginary number.  Imaginary numbers are in the form: For more info on imaginary numbers, check out my lessons on complex numbers. Improper FractionThere's really nothing "improper" about them and we use them in algebra and higher math all the time.  An improper fraction is when the numerator is greater than or equal to the denominator.For more info on improper fractions and how to change them into mixed numbers, check out my lesson onimproper  fractions.  Inequality
An inequality is like an equation, but without the "equals" sign.  Instead, it has one of these signs: Examples:     5 > 3       2x + 7 <  -9 Infinite
An infinite list of numbers goes on forever and ever and never stops.
Example:  The whole numbers:  0, 1, 2, 3, 4, ... Infinity
Here is the official symbol for infinity...  It's a "figure eight" laying on its side.  InscribedExample:  The picture on the right...  The circle is said to be inscribed in the triangle since the circle is inside so that its edge (circumference) is just touching each of the sides of the triangle.  Integer
An integer is one member of this set:  { ... , -3, -2, -1, 0, 1, 2, 3, 4, 5, ... } IntersectionAn intersection is the place where two things cross.  Intersection of Sets
The intersection of two sets is a set of elements that appears in both sets.
Example:    A = { 1, 2, 3, a, b, c }     B = { 3, 4, 5, b, c, f, g }  Inverse - Additive Inverse
The additive inverse of a number, a, is the number, -a, you add to it to get 0 (the additive identity).
+ (-a) = 0
Examples:  The additive inverse of 8 is -8 since 8 + (-8) = 0
The additive inverse of -2 is 2 since -2 + 2 0 Inverse - Multiplicative Inverse
The multiplicative inverse of a number, a, is the number, 1/a, that you multiply it to so you get 1 (the multiplicative identity). Example:  The multiplicative inverse of 5 is 1/5.
The multiplicative inverse of 1/2 is 2. Irrational Number
An irrational number is a number whose decimal part goes on forever and ever without repeating.  Isosceles TrapezoidAn isosceles trapezoid is a trapezoid whose sides are the same length (congruent.) For more info on trapezoids, check out my Properties of Trapezoids page.  Isosceles TriangleAn isosceles triangle is a triangle with two equal sides. For  more info on triangles, check out my Properties of Triangles page. A B C D E F G H I no J's K L M N O P Q R S T U V W no X's no Y's no Z's