Lesson 14Estimating Areas

Learning Goal

Let’s estimate the areas of weird shapes.

Learning Targets

  • I can calculate the area of a complicated shape by breaking it into shapes whose area I know how to calculate.

Warm Up: Mental Calculations

Problem 1

Find a strategy to make each calculation mentally:

Activity 1: House Floorplan

Problem 1

Here is a floor plan of a house. Approximate lengths of the walls are given.

The floorplan of a house, with dimensions and room names

What is the approximate area of the home, including the balcony? Explain or show your reasoning.

Activity 2: Area of Nevada

Problem 1

Estimate the area of Nevada in square miles. Explain or show your reasoning.

A map of Nevada with dimensions 320 mi across, 490 mi vertical along the right side, and 270 mi vertical from the left bottom corner to base of state

Are you ready for more?

Problem 1

The two triangles are equilateral, and the three pink regions are identical. The blue equilateral triangle has the same area as the three pink regions taken together. What is the ratio of the sides of the two equilateral triangles?

An equilateral triangle with 3 pink corners and a blue triangle in the center

Lesson Summary

We can find the area of some complex polygons by surrounding them with a simple polygon like a rectangle. For example, this octagon is contained in a rectangle.

The rectangle is 20 units long and 16 units wide, so its area is 320 square units. To get the area of the octagon, we need to subtract the areas of the four right triangles in the corners. These triangles are each 8 units long and 5 units wide, so they each have an area of 20 square units. The area of the octagon is or 240 square units.

A complex polygon surrounded by a rectangle measuring 16 by 20. Inside dimensions of the polygon are noted.

We can estimate the area of irregular shapes by approximating them with a polygon and finding the area of the polygon. For example, here is a satellite picture of Lake Tahoe with some one-dimensional measurements around the lake.

The area of the rectangle is 160 square miles, and the area of the triangle is 17.5 square miles for a total of 177.5 square miles. We recognize that this is an approximation, and not likely the exact area of the lake.

A satellite picture of Lake Tahoe surround by a rectangle and a triangle. The rectangle measure 20 mi by 6 mi. and the triangle has a base of 5 mi and a height of 7 mi