Unit 1: Day 2

Day 2: Introducing the concept of scale

This lesson will build on day one. Once you know how to find the area of one shape, you can then figure out the area of a larger or smaller similar shape.

The basic concept is that of dilations and ratios. Take this picture for example:

We have the 2 side lengths of the bigger triangle. To find out x, we set up a proportion that looks like this:                                    4  =  2 We'll solve this using the fish method, or cross multiplying. 2x9= 18, then we divide it by
9      x.  4, giving us 4.5cm as x.


You can use this method with any set of 2 numbers. They don't have to be side lengths. You could use area and a side length, area and perimeter, or perimeter and a side length.

Video!! 

This should help with the above if you're still confused.

Quiz Time!

1. What is the area of the bigger shape?

2. Can you only scale using side lengths?

3. What would the dimensions of the rectangle be if you used a dilation factor of 3?

Key

1. The area of the larger triangle is 18 cm². We cross multiplied 6 and 12, and divided by 4. 

2. No, you can scale an object accurately using any 2 measurements.

3. The length would be 15 units and the height would be 9 units.