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The short version A millennium is 1,000 years. The plural is millennia, so 2 millennia = 2,000 years. One millennium = 10 centuries = 100 decades. To convert years to millennia, divide by 1,000. Because the calendar has no year 0, the third millennium technically began on January 1, 2001, even though most of the…
The short version An ellipse is a closed curve where the distances from any point on it to two fixed points (the foci) always add up to the same number. “Oval” is an everyday word for any rounded, egg-like shape. Every ellipse is an oval, but not every oval is an ellipse. Quick test: an…
The short version An ordered pair is two numbers written in parentheses, like (3, 5), that name one exact point on a graph. The first number is x (left or right), the second is y (up or down). Order matters: (3, 5) and (5, 3) are different points. To plot one, start at the origin…
The short version A number is an idea that tells how many, how much, or which position. It’s the amount itself, not the mark on paper. A numeral is the symbol you write for a number (5, five, V). A digit is one of the ten symbols 0 through 9 used to build numerals. The…
The short version The main types of lines in math are horizontal, vertical, diagonal (oblique), parallel, perpendicular, intersecting and skew. Older grades add transversals, concurrent lines, tangents and secants. A line goes on forever in both directions, a ray has one endpoint, and a line segment has two. Parallel lines never meet and have the…
The short version A line of symmetry is a line that splits a shape into two halves that are exact mirror images. Fold along it and the halves match perfectly. A square has 4 lines of symmetry: 2 through opposite corners and 2 through the middles of opposite sides. Any regular polygon has as many…
The short version A sum in math is the answer you get when you add numbers together. In 6 + 9 = 15, the sum is 15. The numbers being added are called addends. You can add two addends or twenty, and the result is still called the sum. “Find the sum” always means add….
The short version A right angle is an angle that measures exactly 90 degrees. It’s the square corner you see on a sheet of paper, a door frame or a book. Diagrams mark right angles with a small square in the corner instead of an arc. If you see that square, the angle is 90°,…
The short version In math, the input is the number you start with and the output is the number you get after applying a rule. If the rule is “add 5” and the input is 3, the output is 8. Kids first meet input and output through function machines and input/output tables in grades 3…
The short version An octagon is a flat shape with 8 straight sides, 8 corners and 8 angles. Any 8 sided shape is an octagon, whether or not the sides are equal. The interior angles of every octagon add up to 1,080°. In a regular octagon, each angle is 1,080 ÷ 8 = 135°. The…