Prior to this lesson, it is likely that students have used the law of sines to solve problems involving certain non-right triangles. During this lesson, students will discover how the law of cosines can be used to solve problems involving non-right triangles for which the law of sines cannot be used.
Introduce the lesson with diagrams such as the following:
Pose these questions to students:
- Are there unique triangles with the side and angle measures given above? Why? [Yes. SSS and SAS are two methods that prove triangles congruent. Therefore, if three sides of a triangle are known, or if two sides and an included angle are known, the triangle is unique.]
- Why can't the law of sines be used to determine the measures of the missing angles and/or sides? [To use the law of sines, you need to know the measure of at least one angle in the triangle. Therefore, the law of sines cannot be used to determine the measures of the missing angles in the triangle with only three sides given. In addition, you need to know the measure of an angle and an opposite side to use the law of sines. Therefore, the law of sines cannot be used to determine the measures of the missing angles and side in a triangle with the given sides and included angle, because the side opposite the given angle is unknown.]
Instruct students to think about the questions individually. Then, allow them to discuss their ideas with a partner. Then discuss the questions with the whole group.
After discussing the questions, distribute a copy of the Law of Cosines activity sheet to each student, and have students read the introduction. After students read the introduction, make the connection between the problems posed at the beginning of class and the purpose of the activity. Explain to students that during this activity, they will develop the law of cosines, which addresses the cases of triangles for which the law of sines cannot be used.
The Law of Cosines activity sheet provides students with specific instructions that guide them through the activity. The teacher will take on different roles as students engage in the activity. Before students begin the questions on the activity sheet, the teacher's role is explainer. To begin, draw triangle ABC with altitude k on the chalkboard or overhead projector, as shown below. Then, describe the diagram to students. Point out how the altitude k is perpendicular to side c.
Ask students why the parts of side c can be represented as x and c – x.
[Since the two parts add to equal c, if we use a variable to represent one of the parts, we know that the other part can be represented as c minus the first part.]
For Questions 1‑8 on the activity sheet, the teacher's role changes to facilitator. Students should work with a partner as they complete these questions. The teacher should circulate around the room to facilitate discussion between students and to answer questions.
To facilitate partner discussion, the teacher should ask the following questions while circulating around the room:
- If k2 = b2 ‑ (c ‑ x)2 and k2 = a2 ‑ x2, what conclusions can be drawn about b2 ‑ (c ‑ x)2 and a2 ‑ x2? [The transitive property allows us to conclude that they must be equal.]
- Why use cos B to eliminate x from the equation b2 = a2 + c2 ‑ 2cx? [Students may be tempted to say that they used cos B because they are developing the law of cosines. However, push students to think more deeply. Cos B is the logical choice because it involves x, which we want to eliminate, and a, which is a side of triangle ABC.]
After students have completed Questions 1‑8, instruct them to post the equations they wrote. In this way, the teacher can check the work and make certain that all students have the correct equations.
After reviewing Questions 1‑8, the teacher's role returns to explainer. Point out to students the different parts of the law of cosines. Explain how the law of cosines written in Question 8 is only one form of the law of cosines, as different angles and sides can be found by substituting different variables into the equation. After explaining, instruct students to work on Question 9 and to check their answers with a partner. Once students have finished, ask them to state the equations they wrote.
After students complete the Law of Cosines activity sheet, have them use the law of cosines to determine the measures of the missing sides and angles in the triangles discussed at the beginning of the lesson. While students are using the law of cosines, the teacher's role again becomes facilitator. The teacher should circulate around the room while students determine the missing measures. (Students may have difficulty solving for the angles at first. The teacher may wish to discuss the algebraic techniques involved before students begin working.) After students have had time to determine the measures and check answers with their partner, the teacher can choose students to post solutions on the board.
The solutions for the triangles are shown above, with the measures of the unknown sides and angles in green.
To conclude the lesson, ask students to describe the relationship between the given sides and/or angles of a triangle that would require the use of the law or sines or the law of cosines to determine the measures of the missing sides and/or angles in the triangle.
[Law of Sines: Two angles and a side (AAS or ASA); two sides and a non-included angle (SSA).
Law of Cosines: Three sides (SSS); two sides and an included angle (SAS).]
