O

obtuse angle

An angle that measures more than 90°.

pages 369–372

obtuse triangle

An obtuse triangle has one obtuse angle.

pages 400, 422

odd number

Numbers that are not multiples of 2 (1, 3, 5, 7, etc.) are called odd numbers.

page 68

ones period

The sequence of digits in the ones place, the tens place, and the hundreds place. In the number 456,789,987 the ones period is in bold type.

pages 220–221, 230

operation

A process that takes two numbers and results in a third. This is called a binary operation. For example, addition, subtraction, multiplication, and division are operations.

pages 107, 252–256

opposite angles

opposite sides

page 420

order of operations

A way to clarify which math procedures should go first in a math sentence that has more than one operation. 1. Do calculations in parentheses first. 2. Do all multiplications and division in order from left to right. 3. Then do all additions and subtractions in order from left to right.

pages 252–256

ordered pair

A pair of numbers that gives the coordinates of a point on a grid in relation to the origin. The horizontal coordinate is given first; the vertical coordinate is given second. For example, the ordered pair (5, 2) tells us to move five units to the right of the origin and 2 units up.

pages 36–39, 57, 187, 190, 195, 248–249, 476, 479, 561–562, 573–574

origin

The point at which the x-axis (horizontal) and y-axis (vertical) intersect on a coordinate plane. The origin is described by the ordered pair (0, 0) and serves as a reference point.

page 37

ounce

A unit for measuring equal to of a pound. There are 16 ounces in a pound.

pages 273, 330, 489–491

output

The information that comes out or results from performing a function.

pages 208–211, 569–570, 577