All about Proportion

Proportions

Proportion says that two ratios (or fractions) are equal.

Example:

So 1-out-of-3 is equal to 2-out-of-6

The ratios are the same, so they are in proportion.

Example: Rope

A ropes length and weight are in proportion.

When 20m of rope weighs 1kg, then:

• 40m of that rope weighs 2kg
• 200m of that rope weighs 10kg
• etc.

So:

201 = 402

Sizes

When shapes are "in proportion" their relative sizes are the same.

Here we see that the ratios of head length to body length are the same in both drawings. So they are proportional. Making the head too long or short would look bad!

Working With Proportions

NOW, how do we use this?

Example: you want to draw the dog's head, and would like to know how long it should be:

Let us write the proportion with the help of the 10/20 ratio from above:

?	=	10
42		20

Now we solve it using a special method:

Multiply across the known corners,
then divide by the third number

And we get this:

? = (42 × 10) / 20 = 420 / 20 = 21

So you should draw the head 21 long.

To know more or to answer questions visit:

http://www.mathsisfun.com/algebra/proportions.html







Taken from MathsisFun