Acute Angle  -  An angle with a measure of less than 90 degrees.

Addition Properties of Inequality  -  If a < b, then a + c < b + c
If a > b, then a + c > b + c

Greater Than  -  a > b if and only if there is a positive number c such that a - c = b.

Inequality  -  A statement that shows the relationship between two (or more) expressions with one of the following five signs: <, ≤, >, ≥, ≠.

Inequality Properties of Opposites  -  If a > 0, then - a < 0
If a < 0, then - a > 0

Less Than  -  a < b if and only if there is a positive number c such that a + c = b.

Multiplication and Division Properties of Inequality  -  For positive numbers:
If a < b and c > 0, then ac < bc and

<

If a > b and c > 0, then ac > bc and

>

For negative numbers:
If a < b and c < 0, then ac > bc and

>

If a > b and c < 0, then ac < bc and

<

Obtuse Angle  -  An angle with a measure greater than 90 degrees, but not more than 180 degrees.

Property of Squares of Real Numbers  -  a²≥ 0 for all real numbers a.

Right Angle  -  An angle with a measure of exactly 90 degrees.

Subtraction Properties of Inequality  -  If a < b, then a - c < b - c
If a > b, then a - c > b - c

Transitive Properties of Inequality  -  If a < b and b < c, then a < c.
If a > b and b > c, then a > c.

Trichotomy Property  -  For any two real numbers a and b, exactly one of the following is true: a < b, a = b, a > b.