Using the Law of Cosines to Find Angle Measures

Oct 13, 2025 by wbrum.org 87 views

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Introduction

In this article, we will explore the problem of finding the measures of angles in a right triangle with known side lengths. We will use the given side lengths to calculate the measures of the angles in the triangle. This problem is a classic example of applying trigonometric concepts to solve a real-world problem.

The Problem

A right triangle has side lengths $AC = 7$ inches, $BC = 24$ inches, and $AB = 25$ inches. We are asked to find the measures of the angles in triangle ABC.

Using the Law of Cosines to Find Angle Measures

To find the measures of the angles in the triangle, we can use the Law of Cosines. The Law of Cosines states that for any triangle with side lengths $a$ , $b$ , and $c$ , and angle $C$ opposite side $c$ , the following equation holds:

c^2 = a^2 + b^2 - 2ab \cos C

We can use this equation to find the measures of the angles in the triangle.

Finding Angle A

First, we will find the measure of angle A. We know that $AC = 7$ inches and $AB = 25$ inches. We can use the Law of Cosines to find the measure of angle A:

25^2 = 7^2 + 24^2 - 2(7)(24) \cos A

Simplifying the equation, we get:

625 = 49 + 576 - 336 \cos A

Combine like terms:

625 = 625 - 336 \cos A

Subtract 625 from both sides:

0 = -336 \cos A

Divide both sides by -336:

\cos A = 0

Since $\cos A = 0$ , we know that angle A is a right angle, or 90 degrees.

Finding Angle B

Next, we will find the measure of angle B. We know that $BC = 24$ inches and $AB = 25$ inches. We can use the Law of Cosines to find the measure of angle B:

25^2 = 24^2 + 7^2 - 2(24)(7) \cos B

Simplifying the equation, we get:

625 = 576 + 49 - 336 \cos B

Combine like terms:

625 = 625 - 336 \cos B

Subtract 625 from both sides:

0 = -336 \cos B

Divide both sides by -336:

\cos B = 0

Since $\cos B = 0$ , we know that angle B is a right angle, or 90 degrees.

Finding Angle C

Finally, we will find the measure of angle C. We know that $AC = 7$ inches and $BC = 24$ inches. We can use the Law of Cosines to find the measure of angle C:

7^2 = 24^2 + 25^2 - 2(24)(25) \cos C

Simplifying the equation, we get:

49 = 576 + 625 - 1200 \cos C

Combine like terms:

49 = 1201 - 1200 \cos C

Subtract 1201 from both sides:

-1152 = -1200 \cos C

Divide both sides by -1200:

\cos C = \frac{1152}{1200}

Simplify the fraction:

\cos C = \frac{48}{50}

Reduce the fraction:

\cos C = \frac{24}{25}

Since $\cos C = \frac{24}{25}$ , we know that angle C is an acute angle.

Calculating the Measure of Angle C

To find the measure of angle C, we can use the inverse cosine function:

m \angle C = \cos^{-1} \left( \frac{24}{25} \right)

Using a calculator, we get:

m \angle C = 36.87^{\circ}

Conclusion

In this article, we used the Law of Cosines to find the measures of the angles in a right triangle with known side lengths. We found that angle A is a right angle, or 90 degrees, angle B is a right angle, or 90 degrees, and angle C is an acute angle with a measure of 36.87 degrees.

Final Answer

The final answer is:

$m \angle A = 90^{\circ}$
$m \angle B = 90^{\circ}$
$m \angle C = 36.87^{\circ}$

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Introduction

In our previous article, we explored the problem of finding the measures of angles in a right triangle with known side lengths. We used the Law of Cosines to calculate the measures of the angles in the triangle. In this article, we will answer some frequently asked questions related to this problem.

Q&A

Q: What is the Law of Cosines?

A: The Law of Cosines is a mathematical formula that relates the side lengths of a triangle to the cosine of one of its angles. It is used to find the measure of an angle in a triangle when the side lengths are known.

Q: How do I use the Law of Cosines to find the measure of an angle?

A: To use the Law of Cosines, you need to know the side lengths of the triangle and the angle you want to find. You can then plug these values into the formula and solve for the angle.

Q: What is the difference between the Law of Cosines and the Law of Sines?

A: The Law of Cosines and the Law of Sines are two different mathematical formulas that relate the side lengths of a triangle to the trigonometric functions of its angles. The Law of Cosines is used to find the measure of an angle when the side lengths are known, while the Law of Sines is used to find the measure of an angle when the side lengths and the sine of the angle are known.

Q: Can I use the Law of Cosines to find the measure of an angle in any triangle?

A: Yes, you can use the Law of Cosines to find the measure of an angle in any triangle, as long as you know the side lengths and the angle you want to find.

Q: What if I don't know the side lengths of the triangle?

A: If you don't know the side lengths of the triangle, you can use other methods to find the measure of the angle, such as the Law of Sines or the Law of Tangents.

Q: Can I use a calculator to find the measure of an angle using the Law of Cosines?

A: Yes, you can use a calculator to find the measure of an angle using the Law of Cosines. Simply plug in the values and solve for the angle.

Q: What if I get a negative value for the cosine of an angle?

A: If you get a negative value for the cosine of an angle, it means that the angle is obtuse (greater than 90 degrees). You can use the inverse cosine function to find the measure of the angle.

Q: Can I use the Law of Cosines to find the measure of an angle in a right triangle?

A: Yes, you can use the Law of Cosines to find the measure of an angle in a right triangle. However, since the triangle is a right triangle, one of the angles is already known to be 90 degrees.

Q: What if I have a triangle with two sides of equal length?

A: If you have a triangle with two sides of equal length, it is an isosceles triangle. You can use the Law of Cosines to find the measure of the angle between the two equal sides.

Conclusion

In this article, we answered some frequently asked questions related to finding the measures of angles in a right triangle with known side lengths. We hope that this article has been helpful in clarifying any doubts you may have had.

Final Answer

The final answer is:

The Law of Cosines is a mathematical formula that relates the side lengths of a triangle to the cosine of one of its angles.
The Law of Cosines can be used to find the measure of an angle in any triangle, as long as you know the side lengths and the angle you want to find.
A calculator can be used to find the measure of an angle using the Law of Cosines.
If you get a negative value for the cosine of an angle, it means that the angle is obtuse (greater than 90 degrees).
The Law of Cosines can be used to find the measure of an angle in a right triangle, but since the triangle is a right triangle, one of the angles is already known to be 90 degrees.