Equilateral Triangle

An equilateral triangle has all 3 sides of equal length. Equilateral triangles are also equiangular with all 3 internal angles equal to 60°.

I​n the pic below, sides a, b and c are equal. Angles A, B and C are all equal to 60°.

Interior vs Exterior

Identifying interior angles within a polygon is rather obvious while the exterior ones can sometimes lead to confusion.

An exterior angle is formed by extending one of the sides of the polygon. The rule to remember is that the pair of interior/exterior angles is supplementary (they add up to 180°).

Alternate Angles

Angles situated on alternate sides of a transversal line crossing two parallel lines are called 'alternate angles'.

Alternate angles are equal.

In the pic below, a° = b° and x° = y°

Pi or π

Pi is the ratio of a circle's circumference (the distance around its edge) to its diameter (the distance across it through its centre).

T​he value of π is irrational (it cannot be expressed as an exact fraction or decimal) so an approximate value is often used in calculations.

Common approximate values used are 22/7 (as a fraction) and 3.142 (as a decimal).

Pandigital Number

A whole number containing each of the digits exactly once (0, 1, 2, 3, 4, 5, 6, 7, 8 and 9).

1023456789 is the smallest pandigital number.

9876543210 is the largest pandigital number.

I play tennis, love means nothing to me.

Dice

Numbers on opposite sides of a dice always add up to seven.

Significant Figures

Digits in a number that provide a good estimation of its size. The first and most significant figure in a number is always the first non-zero digit (reading from left to right).

F​or instance, the first significant figure of 7092 is 7 because it informs us that the number is 7 thousand and something. Although the digit 9 is larger, it only represents 9 tens hence being less significant than the digit 7.

Discount

A deduction from the usual cost of an item.

A​ 20% discount means that the cost has been reduced by 20%.