Intro to Area of Triangles

This is an adaptation of the OUR lesson 6-1.8 Area of Triangles. Student Facing Goals I can use what I know about parallelograms to reason about the area of triangles. Lesson Narrative This lesson builds on students’ earlier work decomposing and rearranging regions to find area. It leads students to see that, in addition to using area-reasoning methods from previous lessons, they can use what they know to be true about parallelograms (i.e. that the area of a parallelogram is ) to reason about the area of triangles. Students begin to see that the area of a triangle is half of the area of the parallelogram of the same height, or that it is the same as the area of a parallelogram that is half its height. They build this intuition in several ways: by recalling that two copies of a triangle can be composed into a parallelogram; by recognizing that a triangle can be recomposed into a parallelogram that is half the triangle’s height; or by reasoning indirectly, using one or more rectangles with the same height as the triangle. They apply this insight to find the area of triangles both on and off the grid.

By Todd Steinhauer