Apply the Law of Sines and the Law of Cosines to solve mathematical and real-world problems involving triangles.

General Information

**Subject Area:**Mathematics (B.E.S.T.)

**Grade:**912

**Strand:**Trigonometry

**Date Adopted or Revised:**08/20

**Status:**State Board Approved

## Benchmark Instructional Guide

### Connecting Benchmarks/Horizontal Alignment

### Terms from the K-12 Glossary

- Angle
- Triangle

### Vertical Alignment

Previous Benchmarks

Next Benchmarks

### Purpose and Instructional Strategies

In grade 8, students solved problems involving triangles using Pythagorean Theorem, Triangle Sum Theorem and Triangle Inequality Theorem. In Geometry, students apply the Law of Sines and the Law of Cosines to solve problems involving right and non-right triangles. In later courses, students will use other trigonometric relationships and identities to solve real-world and mathematical problems involving sides and angles of triangles. Additionally, students will use knowledge of the Law of Sines and Law of Cosines when learning about vectors.- Within the Geometry course, the expectation is to use angle measures given in degrees and not in radians. Additionally, it is not the expectation for students to master the trigonometric ratios of secant, cosecant and cotangent within this course.
- It is customary to use Greek letters to represent angle measures (e.g., θ, α, β, γ).
- The Law of Sines states that given a triangle with side lengths $a$, $b$ and $c$, and angle measures α, β and γ (opposite to the sides respectively), then $\frac{\text{a}}{\text{sin \alpha}}$ = $\frac{\text{b}}{\text{sin \beta}}$ = $\frac{\text{c}}{\text{sin \gamma}}$ or $\frac{\text{sin \alpha}}{\text{a}}$ = $\frac{\text{sin \beta}}{\text{b}}$ = $\frac{\text{sin \gamma}}{\text{c}}$.
- Instruction includes making the connection between the Law of Sines and the congruence
criteria of Angle-Side-Angle and Angle-Angle-Side.
- For example, when given two angle measures and the included side length, students can use the Law of Sines to determine the missing side lengths and angle measures and justify that there is one unique triangle as indicated by the Angle-Side-Angle congruence criterion.

- Instruction includes making the connection between the Law of Sines and the Side-Side-Angle criterion, which is referred to as the ambiguous case.
- For example, when given two side lengths and an angle measure of an opposite angle students can use the Law of Sines to determine the value of the sines of the missing angle measures. If the value of either of the sines is greater than one, then no triangle exists with those angle measures. If the value of both of the sines is less than one, then there are two possible triangles, one acute and one obtuse. If the value of either of the sines is equal to one, then there is exactly one triangle, a right triangle, with those measures.

- The Law of Cosines states that given a triangle with side lengths $a$, $b$ and $c$, and angle
measures α, β and γ (opposite to the sides respectively), then $a$
^{2}= $b$^{2}+ $c$^{2}− 2$b$$c$ cos α, $b$^{2}= $a$^{2}+ $c$^{2}− 2$a$$c$ cos β and $c$^{2}= $a$^{2}+ $b$^{2}− 2$a$$b$ cos γ. - Instruction includes making the connection between the Law of Cosines and the
congruence criteria of Side-Side-Side and Side-Angle-Side.
- For example, when given three side lengths, students can use the three versions of the Law of Cosines to determine the missing angle measures and justify that there is one unique triangle as indicated by the Side-Side-Side congruence criterion. However, if the side lengths do not satisfy the Triangle Inequality Theorem, then no triangle exists.

- Instruction includes making the connection between the Law of Cosines and the Side-Side-Angle criterion, which is referred to as the ambiguous case.
- For example, when given two side lengths and an angle measure of an opposite angle students can use one of the versions of the Law of Cosines to obtain a quadratic equation for the missing side length. If there is no real solutions, then no triangle exists. If there are two real solutions, then there are two possible triangles, one acute and one obtuse. If there is exactly one real solution, then there is exactly one triangle, a right triangle.

- Instruction includes the understanding that the Pythagorean Theorem is a special case of
the Law of Cosines when the included angle measure is 90°, whose cosine is zero.
*(MTR.5.1)* - Instruction includes the understanding that sines and cosines of obtuse angles are necessary when solving problems involving obtuse triangles. Enrichment of this benchmark includes the connection to the unit circle.

### Common Misconceptions or Errors

- Students may have trouble keeping track of which sides or angles belong in the Laws of Sines and Cosines.

### Instructional Tasks

*Instructional Task 1 (MTR.3.1, MTR.4.1)*

- Triangle
*ABC*is an acute triangle. The measure of*AB*is 17.8 inches, the measure of*BC*is 16.5 inches and the measure of angle*C*is 75°.- Part A. Determine the measure of angle
*A*. Compare your method used with a partner. - Part B. Determine the measure of angle
*B*.

- Part A. Determine the measure of angle

Instructional Task 2 (

Instructional Task 2 (

*MTR.4.1*,*MTR.5.1*)- Part A. Given side lengths 10, 11 and 22, use the Law of Cosines to determine if a triangle exists with those side lengths.
- Part B. How could you determine, without using the Law of Cosines, whether such a triangle exists?
- Part C. If the triangle from Part A does not exist, change one of its side lengths so that a triangle does exist.

### Instructional Items

*Instructional Item 1*

- Find the remaining measures of the angles and sides of triangle
*ABC*.

**The strategies, tasks and items included in the B1G-M are examples and should not be considered comprehensive.*

## Related Courses

This benchmark is part of these courses.

1202340: Precalculus Honors (Specifically in versions: 2014 - 2015, 2015 - 2022, 2022 and beyond (current))

1206320: Geometry Honors (Specifically in versions: 2014 - 2015, 2015 - 2022, 2022 and beyond (current))

## Related Access Points

Alternate version of this benchmark for students with significant cognitive disabilities.

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## Lesson Plans

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## Problem-Solving Task

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