1. Proportions

2. A proportion is an equation stating that two ratios are equivalent.
In a proportion, a cross product is the product of the numerator of one ratio and the denominator of the other ratio.
If the cross products of two ratios are equal, then the ratios form a proportion.

3. Example:
Determine whether 2/3 and 10/15 form a proportion.
2 ? 10
Cross multiply: 2 x 15 = 30 and 3 x 10 = 30, so they do form a proportion.

4. You can use cross products to find a missing term in a proportion
You can use cross products to find a missing term in a proportion. This is known as solving the proportion. Solving a proportion is similar to solving an equation.
Ex: n = 3
8 4
Cross multiply: 4n = 24, then solve as an equation.

5. A scale drawing represents something that is too big or too small to be drawn at actual size. A map is an example of a scale drawing. The scale gives the relationship between the distance on the map and the actual distance. A proportion can be used to find the actual distance between two places.

6. Example:
On a map, 1 cm = 10 miles. If two cities are 20.5 cm apart on the map, we set up the proportion this way:
1cm = 20.5cm
10m x m
Solve: 1x = 205, so the two cities are 205 miles apart.