
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
a^{2}≥ 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.