# Lesson 15 — Activity 3: Estimating and Calculating the Area of Triangles

#### A triangle is a closed two-dimensional shape consisting of three straight sides. In this activity, you will estimate and calculate the area of triangles. #### The area of a triangle can be estimated using a variety of methods. One method is to use dot paper. #### Estimate the area of the following triangle. Remember to count the number of whole squares, then combine partial squares to make whole squares. Write your answer as in the above example. #### Calculating the Area of Triangles If you fold a square or a rectangle in half diagonally, you can make two triangles. Take some paper and try this. #### Therefore, the area of a triangle must be half the area of a square or rectangle. The formula for calculating the area of a triangle is: #### Let's say you had a triangle with a base that was 14 mm long and a height that was 17 mm. You could calculate the area like this: #### Calculate the area of the triangles below. Follow the formula above.

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