T

ten thousandths

1 of 10 thousand equal parts of a whole (.0001).

pages 391–392

tenths

1 of 10 equal parts of a whole (0.1).

pages 63, 77, 373–374, 388, 391–392, 411–414, 477

thermometer

An instrument for measuring and indicating temperature.

pages 244, 247

thousandths

1 of 1000 equal parts of a whole (.001).

pages 369–370, 373–374, 391–392

trapezoid

1. A quadrilateral with exactly one pair of parallel sides.
2. A quadrilateral with at least one pair of parallel sides.

pages 281, 286–287

triangle

A polygon with three sides.

pages 188, 194, 279, 283–285

true number sentence

A number sentence where the amount on one side of the equal sign has the same value as the amount on the other side;
e.g., 1000 + 30 + 2 = 1000 + 10 + 10 + 10 + 2.

page 377

turn-around facts

Addition or multiplication facts that have the same numbers but in a different order (e.g., 3 + 4 = 7 and 4 + 3 = 7, 3 × 4 = 12 and 4 × 3 = 12). (See also commutative property of multiplication.)

page 54

turn-around rule

A term used to describe the commutative property of addition
(e.g., 4 + 6 = 6 + 4) and multiplication (e.g., 4 × 6 = 6 × 4).
(See also commutative property of multiplication.)

pages 63, 77, 373–374, 388, 391–392, 411–414, 477

twin primes

A pair of prime numbers whose difference is 2. For example, 3 and 5 are twin primes.

page 431