Overview for the topic:
- learn how to write and simplify ratios
- learn how to determine whether two ratios are a proportion and how to use proportions to solve problems learn how to determine whether two triangles are similar

A ratio is the comparison of two numbers using division.
The ratio of x to y can be written or x:y.
Ratios are usually expressed in simplest form.

What is a
Since 2/4 and 3/6 are both equal to 1/2, they are equal to each other.
A statement that two ratios are equal is called a proportion.
A proportion can be written in one of the following ways:
2/4 =3/6 or 2:4 = 3:6

The first and last numbers in a proportion are called the extremes.
The middle numbers are called the means.
Proportion

Means-Extremes Property
In a proportion, the product of the means equals the product of the extremes.
Proportion

Two figures are similar if you can show that the following two statements are true:
1. Corresponding angles are congruent.
2. Corresponding sides are in proportion.

Examples:

SOURCE: http://www.education.com/study-help/article/ratioproportion-similarity/