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EnchantedLearning.com Math Dictionary

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R is for ...

radian A radian is a unit of angular measure that is equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle. A radian is about 57.2957° (1 degree = 0.0174532 radians). There are 2*pi radians in 360°.

radius The radius is the distance from the center of a circle (or a sphere) to the edge.

1/2 ratio A ratio is the relationship between two numbers, denoted by division. For example, if the ratio of boys to girls at a school is 4 to 5, this can be written as 4/5 or 4:5.

rational number A rational number is a number that can be expressed in the form a/b, where a and b are integers and b≠0. For example, 0, 1/2, and -41 are rational numbers. Each rational number corresponds to a unique point on the number line (but most points on the number line do not correspond to a rational number -- they correspond to an irrational number).

ray A ray (also called a half line) is a straight line that extends infinitely far in one direction from a point.

real number Real numbers are all rational and irrational numbers (but not the imaginary numbers). The real numbers represent all the points on the number line.

rectangle A rectangle is a four-sided figure (a quadrilateral) whose sides are at right angles to each other. A square is a type of rectangle whose four sides are all the same length.

regular polygon A regular polygon is a polygon (a polygon is a many-sided figure with straight edges) whose sides are all the same length and whose interior angles are all the same (like an equilateral triangle or a square).

remainder The remainder is the part part of the answer in long division that does not go evenly into the divisor. The dividend divided by the divisor is the quotient plus the remainder.

rhomboid A rhomboid is a parallelogram with oblique angles and adjacent sides have different lengths.

rhombus A rhombus is a parallelogram with equal-length sides.

right angle A right angle looks like the corner of a square; it extends ninety degrees. Each of the interior angles of a square is a right angle.

right triangle A right triangle has one angle that is a right angle (extending 90 degrees).

Roman numerals Thousands of years ago, in ancient Rome, the Romans used a system of numbers that we call Roman numerals. In this system, I=1, V=5, X=10, L=50, C=100, D=500, and M=1,000. Roman numerals express numbers as sums and differences. For example, 6 is VI (five + one), but nine is IX (ten - one). In general, when a smaller Roman numeral follows a larger numeral, you add the numbers (for example, XII is ten +one+one=twelve). When a larger numeral follows a smaller one, you subtract the numbers (for example, IV is five-one=four, and XL is 50-10=40).

round Round things are curved. A circle is round.

Number Nearest ten Nearest hundred Nearest thousand 368 370 400 0 1,234 1,230 1,200 1,000 89,355 89,360 89,400 89,000 rounding When a number is rounded (or rounded off), it is approximated by eliminating the least significant digits. Whole numbers can be rounded to the tens place, hundreds place, thousands place, and so on. Decimals can also be rounded, estimating the number to the nearest tenth, hundredth, thousandth, and so on. Rounding is used to make a number easier to work with. For example, if you know that there are 496 students in your school, you can say that there are approximately 500 students in your school.

ruler A ruler is a long, straight object that is used for measuring distances or drawing straight lines.

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EnchantedLearning.com Math Dictionary

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Counting

Addition

Subtraction

Multiplication

Division

Number Line

Fractions

Decimals

Rounding

Graphs

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