Operations that, when they are combined, leave the entity on which they operate unchanged. Examples: addition and subtraction are inverse operations e.g. 5 + 6 - 6 = 5. Multiplication and division are inverse operations e.g. 6 × 10 / 10 = 6.

Some operations, such as reflection in the x-axis, are self-inverse.

A number that is not an integer and cannot be expressed as a common fraction with a non-zero denominator. Real irrational numbers, when expressed as decimals, are infinite, nonrecurring decimals.

Examples: the square root of three. Pi is also an irrational number.

A triangle in which two sides have the same length and consequently two angles are equal. This definition includes an equilateral triangle as a special case.