Lesson 9
Formula for the Area of a Triangle
Lesson Narrative
In this lesson students begin to reason about area of triangles more methodically: by generalizing their observations up to this point and expressing the area of a triangle in terms of its base and height.
Students first learn about bases and heights in a triangle by studying examples and counterexamples. They then identify base-height measurements of triangles, use them to determine area, and look for a pattern in their reasoning to help them write a general formula for finding area (MP8). Students also have a chance to build an informal argument about why the formula works for any triangle (MP3).
Learning Goals
Teacher Facing
- Compare, contrast, and critique (orally) different strategies for determining the area of a triangle.
- Generalize a process for finding the area of a triangle, and justify (orally and in writing) why this can be abstracted as $\frac12 \boldcdot b \boldcdot h$.
- Recognize that any side of a triangle can be considered its base, choose a side to use as the base when calculating the area of a triangle, and identify the corresponding height.
Student Facing
Let’s write and use a formula to find the area of a triangle.
Required Materials
Learning Targets
Student Facing
- I can use the area formula to find the area of any triangle.
- I can write and explain the formula for the area of a triangle.
- I know what the terms “base” and “height” refer to in a triangle.
CCSS Standards
Glossary Entries
-
opposite vertex
For each side of a triangle, there is one vertex that is not on that side. This is the opposite vertex.
For example, point \(A\) is the opposite vertex to side \(BC\).
