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Radius- A line segment from the middle of a circle (or sphere) to any point on the circle (or sphere). The radius is also one-half of the diameter (look in D reference section).

Random number- A number that has the same chance of appearing as any other number.

Range- The difference between the maximum and minimum in a set of data.

Rate- A comparison of 2 quantities that have different units.

Rate diagram- A diagram that models rate situations.

Ratio- A comparison by the division, with like, or the same units. Ratios can be expressed by fractions, percents, decimals or words. They can also be written with a colon in between the 2 numbers. The first number is that number out of the second number. Example: The team won 3 out of its 8 ping-pong contests, the ratio would be 3/8 (fraction), 0.375 (decimal), 37 1/2% (percent), 3 to 8, or 3:8.

Ray- A straight line that goes on, or extends infinitely from a point which is its endpoint.

Real number- Any rational or irrational number.

Rectangle- A parallelogram with 4 right angles.

Reduce- 1. To make an object or shape smaller. 2. To put a fraction in simpler form.

Reflection- The “flipping” of a figure over the line of reflection, so that its image is the mirror image of the original. Same as flip.

Regular polygon- A polygon with each side being the same length and each angle having the same measure.

Regular polyhedron- A closed 3-dimensional figure and each face that are formed by a single regular polygon (see above) and each vertex looks the same as every other vertex.

Regular tessellation- A tessellation made up of only one kind of regular polygon (see above).

Remainder- The number or amount left over after one number gets divided by another number.

Repeating decimal- A decimal of which one digit or a group of digits are repeated. Example: pi, which is 3.1415926535879323846264338327950288419717693993751…

Rhombus- A quadrilateral whose sides are all the same length.

Right angle- A 90° angle.

Right triangle- A triangle which has a right angle (above).

Rotation- A movement if a figure around a fixed point or axis: also known as a turn.

Round- To adjust a number to make it easier to work with. Example: Pi (3.1415926535879323846264338327950288419717693993751…) rounded would be 3.14.