# Online Math Dictionary: R

Easy to understand math definitions for K-Algebra mathematics
Just scroll down or click on the word you want and I'll scroll down for you!

 radian radical radicand radius of a circle ratio rational number rationalizethe denominator ray reciprocalof a number rectangle rectangularcoordinate system regular polygon relation relatively prime remainder repeating decimal rhombus right angle right triangle root of  a polynomial

 RadianA radian is a special unit of measure (like degrees) that measures things that are circular  -- like angles that sweep  around!  You start using radians in trigonometry.To find out more info about radians, check out my Trigonometry of Circlespage.   Radical
A radical is a root (like a square root) of a number.  A radical is made up ofradical sign and something inside called the radicand. Example: To learn more about radicals, check out my Radicals Review lesson. RadicandThe radicand is the thing inside the radical sign. Radius of a CircleThe radius of a circle is a line segment starting at the center and going to the outer edge of thecircle.  The radius is half of the diameter.For more info on circles, check out my Geometry Reference Area.  Ratio
The ratio you've probably heard the most is "miles per hour"...  This is really a fraction with miles in the numerator andhours in the denominator.  In the example below, if you've driven 180 miles in 3 hours, that's a ratio of 60 miles perhour.  Rational Number
A rational number is a number that can be written as a fraction with integers as the numerator and denominator.   This includes fractionsregular counting numbersnegative numbersterminating decimals and repeating decimals.  Rationalize the Denominator
Rationalizing the denominator is a process that gets rid of the radical in the denominator.  Examples: To learn how to rationalize the denominator, check out my Rationalizing Denominators lesson. RayA ray is a partial line that starts at one point and goes on forever and ever in one direction.  Real Numbers
The set of Real Numbers consists of both the rational and irrational numbers. Reciprocal of a Number
The reciprocal of a number is the number you need to multiply by to get 1 as the answer.  The numbers below are reciprocals of each other.  RectangleA rectangle is a quadrilateral whose interior angles are all 90 degrees (all right angles). For more info and properties, check out my Properties of Rectangles page.  Rectangular Coordinate SystemThe rectangular coordinate system is a plane (meaning that it's flat) made up of an x axis (the horizontal line) and a y axis (the vertical line). Coordinates on the rectangular coordinate system are a set of numbers officially called "an ordered pair" that are in the form ( x , y ) ...  The x guy is how far to the right or left you've counted over... and the y guy is how far up or down you've counted. For more info on graphing with rectangular coordinates, check out my Plotting Points lesson.  Regular Polygon
Regular polygons are polygons whose sides are the same length and whose angles are the same size.  Below is aregular triangle (which is called an equilateral triangle), a regular quadrilateral (which is a square), a regular pentagon, a regular hexagon and a regular heptagon.  Relation
A relation is an ordered pair in the form:  (x, y)     Example:  (2, -3)
Ordered pairs are used as rectangular coordinates. Relatively Prime
Two numbers are relatively prime if they have no common factors other than the number 1.  Another way to define it is to say that two numbers are relatively prime if their greatest common divisor is 1.
Examples: 9 and 14 are relatively prime since their greatest common divisor is 1...   4 and 18 are NOT relatively prime since their greatest common divisor is 2. Remainder
The remainder is what might be "left over" when you are doing a division problem.  For example, 3 goes into 17 five times with 2 left over...  So, the remainder is 2.  We write this as  Repeating Decimal
A repeating decimal is a decimal where the decimal part repeats.  This means that  one or more numbers just keep showing up over and over again.   We put a bar over the repeating part: Repeating decimals can always be converted into fractions.  Repeating decimals are a rational numbers. RhombusA rhombus is a quadrilateral whose sides are all the same length (congruent) and whoseopposite interior angles are the same size (congruent.)  Really, a rhombus is just a square that someone sat on and squished!  (ouch) For more info and properties, check out my Properties of the Rhombus page.  Right AngleA right angle is an angle whose measure is 90 degrees. The symbol always used to show that an angle really IS a right angle, is a little square in the corner.  In a geometry problems, if the angle LOOKS like it's 90 degrees, but there is no "square symbol," then, it's NOT really 90 degrees!  This is a very common trick among geometry teachers and people who make those "standardized" tests!  So... beware.  Right TriangleA right triangle is triangle that has a right (90 degree) angle.  Root of a Polynomial
The roots of a polynomials are the solutions when the polynomial is set equal to zero.  This is a bit too complicated for a short definition, so check out my Zeros of a Polynomials lesson.

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