# Solving Right Triangles

Solving right triangles involves determining the lengths of sides and the measures of angles using trigonometric ratios. The Pythagorean theorem is essential: \$$a^2 + b^2 = c^2\$$, where \$$c\$$ is the hypotenuse. Key trigonometric functions—sine, cosine, and tangent—relate the angles to the sides, aiding in solving these triangles effectively.

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## Understanding Right Triangles

Understanding right triangles is fundamental in geometry and trigonometry. Right triangles are unique because they have one angle that is exactly 90 degrees.

### Definition of a Right Triangle

Right Triangle: A right triangle is a triangle in which one of the angles is a right angle (90 degrees). The side opposite the right angle is called the hypotenuse, and the other two sides are called the legs.

### Properties of Right Triangles

Right triangles have some specific properties that you should be aware of for solving problems:

• The sum of the two non-right angles is 90 degrees.
• The hypotenuse is always the longest side.
• Pythagoras' theorem applies to right triangles.
Using these properties, you can solve right triangles by finding unknown sides and angles.

### Pythagoras' Theorem

Pythagoras' Theorem: In a right triangle, the square of the length of the hypotenuse (\textit{c}) is equal to the sum of the squares of the lengths of the other two sides (\textit{a} and \textit{b}). This can be written as: $$c^2 = a^2 + b^2$$.

Example: If the lengths of the legs of a right triangle are 3 and 4, you can find the length of the hypotenuse by using Pythagoras' theorem. $$c^2 = 3^2 + 4^2$$ $$c^2 = 9 + 16$$ $$c^2 = 25$$ $$c = 5$$ Hence, the hypotenuse is 5 units long.

The history of Pythagoras' theorem dates back to ancient civilizations. The Egyptians used a rope loop divided into 12 equal spaces, forming a 3-4-5 triangle, which they used to create right angles for their constructions.

Hint: Always make sure to identify the right angle first; it helps in recognising the hypotenuse and applying the theorem correctly.

### Trigonometric Ratios in Right Triangles

Trigonometric ratios are essential tools for solving right triangles. These ratios relate the angles of a right triangle to the lengths of its sides. The primary trigonometric ratios are sine (\textit{sin}), cosine (\textit{cos}), and tangent (\textit{tan}). They are defined as follows: $$\sin\theta = \frac{\text{opposite}}{\text{hypotenuse}}$$ $$\cos\theta = \frac{\text{adjacent}}{\text{hypotenuse}}$$ $$\tan\theta = \frac{\text{opposite}}{\text{adjacent}}$$

## Steps to Solve Right Triangles

Solving right triangles involves finding the unknown sides and angles. By following a systematic approach, you can solve these triangles effectively.

### Identifying Triangle Parts

The first step in solving a right triangle is identifying its parts. A right triangle consists of three main parts: the hypotenuse, the opposite side, and the adjacent side.

• Hypotenuse: The longest side opposite the right angle.
• Opposite side: The side opposite the angle you are examining.
• Adjacent side: The side next to the angle you are examining and not the hypotenuse.
Recognising these parts is crucial for applying various formulas.

Hint: Label each side clearly when analysing the triangle to avoid confusion.

### Using Pythagoras Theorem

Pythagoras' Theorem: In a right triangle, the square of the length of the hypotenuse $$c$$ is equal to the sum of the squares of the lengths of the other two sides $$a$$ and $$b$$. This can be written as: $$c^2 = a^2 + b^2$$.

Example: If the lengths of the legs of a right triangle are 5 and 12, you can find the length of the hypotenuse by using Pythagoras' theorem. $$c^2 = 5^2 + 12^2$$ $$c^2 = 25 + 144$$ $$c^2 = 169$$ $$c = 13$$ Hence, the hypotenuse is 13 units long.

Pythagoras' theorem also has a rich historical background. It was not only known to Greek mathematicians but was also used in Babylon and ancient India, showing the universality of mathematical principles across different cultures and eras.

### Applying Trigonometric Ratios

Trigonometric ratios are fundamental in linking the angles of a right triangle with the lengths of its sides. The primary trigonometric ratios are: Sine (sin): $$\sin \theta = \frac{\text{opposite}}{\text{hypotenuse}}$$ Cosine (cos): $$\cos \theta = \frac{\text{adjacent}}{\text{hypotenuse}}$$ Tangent (tan): $$\tan \theta = \frac{\text{opposite}}{\text{adjacent}}$$ These ratios help in finding unknown side lengths and angles depending on the information given.

Example: Suppose you know one angle (other than the right angle) of a right triangle is 30 degrees and the length of the hypotenuse is 10 units. To find the length of the opposite side, use the sine ratio. $$\sin 30^\circ = \frac{\text{opposite}}{10}$$ $$\frac{1}{2} = \frac{\text{opposite}}{10}$$ $$\text{opposite} = 10 \times \frac{1}{2}$$ $$\text{opposite} = 5$$ Hence, the length of the opposite side is 5 units.

### Finding Missing Angles

To find the missing angles in a right triangle, you can use the trigonometric ratios or the sum of angles in a triangle property. The sum of angles in any triangle is 180 degrees. Since one angle is already 90 degrees, the sum of the other two angles must be 90 degrees. If you know one of the non-right angles, subtract it from 90 degrees to find the other angle.

Example: If one of the non-right angles in a right triangle is 45 degrees, you can find the other angle as follows: $$90^\circ - 45^\circ = 45^\circ$$ Where both angles are 45 degrees, you have an isosceles right triangle.

Hint: Knowing just one angle other than the right angle can help you determine the remaining angle and side lengths quickly.

## Right Triangle Trigonometry Techniques

In this section, you'll learn about various techniques to solve right triangles using trigonometry. These methods will help in finding unknown sides and angles efficiently.

### Sine, Cosine, and Tangent

Sine, cosine, and tangent are fundamental trigonometric ratios utilised to solve right triangles. Each ratio links an angle of a right triangle to the lengths of its sides.

Trigonometric Ratios:

• Sine (sin): $$\sin\theta = \frac{\text{opposite}}{\text{hypotenuse}}$$
• Cosine (cos): $$\cos\theta = \frac{\text{adjacent}}{\text{hypotenuse}}$$
• Tangent (tan): $$\tan\theta = \frac{\text{opposite}}{\text{adjacent}}$$
Understanding these ratios is crucial for solving any problem involving right triangles.

Example: Given a right triangle where the length of the adjacent side is 8 units and the angle is 60 degrees, find the hypotenuse.Using the cosine ratio: $$\cos 60^\circ = \frac{\text{adjacent}}{\text{hypotenuse}}$$ $$\frac{1}{2} = \frac{8}{\text{hypotenuse}}$$ $$\text{hypotenuse} = 8 \times 2$$ $$\text{hypotenuse} = 16$$ Hence, the hypotenuse is 16 units long.

### Special Right Triangles

Special right triangles have particular angle measures that make calculations easier. The two main types are the 45-45-90 triangle and the 30-60-90 triangle. Each has specific properties and side ratios.

45-45-90 Triangle:

• Each leg is congruent.
• The hypotenuse is $$\sqrt{2}$$ times the length of each leg.
30-60-90 Triangle:
• The length of the hypotenuse is twice the length of the shorter leg.
• The length of the longer leg is $$\sqrt{3}$$ times the length of the shorter leg.

Example: For a 45-45-90 triangle with each leg measuring 5 units, find the hypotenuse: Using the properties of the 45-45-90 triangle: $$\text{hypotenuse} = 5 \times \sqrt{2}$$ Hence, the hypotenuse is approximately 7.07 units.

### Inverse Trigonometric Functions

Inverse trigonometric functions are used to find the angles when the side lengths are known. The main inverse functions are arcsine ($$\sin^{-1}$$), arccosine ($$\cos^{-1}$$), and arctangent ($$\tan^{-1}$$). These functions are the inverses of the basic trigonometric ratios.

Inverse Trigonometric Functions:

• Arcsine ($$\sin^{-1}$$): If $$\sin \theta = x$$, then $$\theta = \sin^{-1} x$$.
• Arccosine ($$\cos^{-1}$$): If $$\cos \theta = x$$, then $$\theta = \cos^{-1} x$$.
• Arctangent ($$\tan^{-1}$$): If $$\tan \theta = x$$, then $$\theta = \tan^{-1} x$$.

Example: Given a right triangle where the opposite side is 7 units, and the hypotenuse is 25 units, find the angle $$\theta$$ using the arcsine function.Using the arcsine function: $$\sin \theta = \frac{7}{25}$$ $$\theta = \sin^{-1} \frac{7}{25}$$Hence, $$\theta \approx 16.26^\circ$$.

Hint: Inverse trigonometric functions are helpful when you need to determine an angle from side lengths.

Radians and degrees are two units to measure angles. In trigonometry, radians often replace degrees because they make calculations involving $$\pi$$ simpler. One complete revolution is $$2\pi$$ radians or 360 degrees.

## How to Solve a Right Triangle: Problem Examples

Solving right triangles involves using various methods and mathematical theorems. In this section, you'll explore specific problem examples to enhance your understanding.

### Using Given Angles

When working with given angles in a right triangle, you can use trigonometric ratios to find the unknown side lengths. The primary trigonometric ratios are sine, cosine, and tangent.

Example: If a right triangle has one angle measuring 45 degrees and the hypotenuse is 10 units, you can find the lengths of the other sides using the sine or cosine ratios.For the opposite side:$$\sin 45^\circ = \frac{\text{opposite}}{10}$$$$\frac{1}{\sqrt{2}} = \frac{\text{opposite}}{10}$$$$\text{opposite} = 10 \cdot \frac{1}{\sqrt{2}}$$$$\text{opposite} \approx 7.07$$For the adjacent side:$$\cos 45^\circ = \frac{\text{adjacent}}{10}$$$$\frac{1}{\sqrt{2}} = \frac{\text{adjacent}}{10}$$$$\text{adjacent} = 10 \cdot \frac{1}{\sqrt{2}}$$$$\text{adjacent} \approx 7.07$$

Hint: Remember that a 45-degree right triangle is isosceles. Both legs are equal.

Different approaches can be used to verify your results, such as checking that the sum of the squares of the two legs equals the square of the hypotenuse. This approach helps to affirm the accuracy of your calculations.

### Working with Given Side Lengths

When you know the lengths of certain sides in a right triangle, you can use various methods, such as Pythagoras' Theorem and trigonometric ratios, to find the remaining sides and angles.

Example: Given a right triangle where one leg is 6 units long, and the hypotenuse is 10 units long, find the length of the other leg.Using Pythagoras' Theorem:$$c^2 = a^2 + b^2$$$$10^2 = 6^2 + b^2$$$$100 = 36 + b^2$$$$b^2 = 64$$$$b = 8$$Hence, the other leg is 8 units long.

Hint: Ensure your calculator is in the correct mode (degree/radian) when working with trigonometric functions.

Understanding the historical context of Pythagoras' Theorem enriches your comprehension. Ancient mathematicians, including Pythagoras, developed this fundamental theorem that has wide applications in geometry and trigonometry.

### Mixed Information Problems

Sometimes, you may encounter problems where both angles and side lengths are provided. Use a combination of Pythagoras' Theorem and trigonometric ratios to solve these problems.

Example: In a right triangle, if one angle is 30 degrees and the length of the adjacent side is 5 units, find the lengths of the hypotenuse and the opposite side.Using the cosine ratio for the hypotenuse:$$\cos 30^\circ = \frac{\text{adjacent}}{\text{hypotenuse}}$$$$\frac{\sqrt{3}}{2} = \frac{5}{\text{hypotenuse}}$$$$\text{hypotenuse} = \frac{5 \cdot 2}{\sqrt{3}}$$$$\text{hypotenuse} \approx 5.77$$Using the sine ratio for the opposite side:$$\sin 30^\circ = \frac{\text{opposite}}{\text{hypotenuse}}$$$$\frac{1}{2} = \frac{\text{opposite}}{5.77}$$$$\text{opposite} = 5.77 \cdot \frac{1}{2}$$$$\text{opposite} \approx 2.89$$

Hint: Double-check your ratios and ensure that the sides and angles correspond correctly in your trigonometric functions.

In some complex problems, you might need to solve for more than one unknown variable. Using systems of equations or simultaneous trigonometric functions can help achieve this. These problems require careful analysis and a step-by-step approach to ensure accuracy.

## Solving Right Triangles - Key takeaways

• Solving Right Triangles: Involves determining unknown angles and sides using specific methods and mathematical theorems.
• Steps to Solve Right Triangles: Systematically identifying parts of the triangle (hypotenuse, opposite, adjacent) and applying appropriate formulas.
• Pythagoras' Theorem: Key theorem stating that the square of the hypotenuse equals the sum of the squares of the other two sides (\textit{c^2 = a^2 + b^2}).
• Trigonometric Ratios: Essential for solving right triangles, include sine (\textit{sin θ = opposite/hypotenuse}), cosine (\textit{cos θ = adjacent/hypotenuse}), and tangent (\textit{tan θ = opposite/adjacent}).
• Example Problems: Practical illustrations demonstrating how to use Pythagoras' Theorem and trigonometric ratios to solve for unknowns.

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What are the primary methods for solving right triangles?
The primary methods for solving right triangles are using the Pythagorean theorem, trigonometric ratios (sine, cosine, and tangent), and the properties of special right triangles (30-60-90 and 45-45-90). Additionally, inverse trigonometric functions can find angles when side lengths are known.
What are the trigonometric ratios used in solving right triangles?
The trigonometric ratios used in solving right triangles are sine (sin), cosine (cos), and tangent (tan). These ratios are defined as follows: sin(θ) = opposite/hypotenuse, cos(θ) = adjacent/hypotenuse, and tan(θ) = opposite/adjacent.
What is the Pythagorean Theorem and how is it used to solve right triangles?
The Pythagorean Theorem states that in a right-angled triangle, the square of the hypotenuse's length is equal to the sum of the squares of the other two sides' lengths: \$$c^2 = a^2 + b^2\$$. To solve right triangles, use this formula to find an unknown side length when the other two side lengths are known.
What are the common mistakes to avoid when solving right triangles?
Common mistakes include: misidentifying the hypotenuse, incorrectly applying trigonometric ratios, neglecting to use the Pythagorean theorem, and errors in angle measurement or conversion between degrees and radians. Always double-check assignments of sides and angles, and verify calculations.
How do you determine which trigonometric ratio to use when solving a right triangle?
To determine which trigonometric ratio to use, identify the given information and what you need to find. Use sine (sin) for the ratio of opposite side to hypotenuse, cosine (cos) for adjacent side to hypotenuse, and tangent (tan) for opposite side to adjacent side. Choose the ratio involving the known and unknown sides or angles.

## Test your knowledge with multiple choice flashcards

How do you find the opposite side's length if you know an angle of 30° and the hypotenuse is 10 units?

In a right triangle, if one angle is 30 degrees and the length of the adjacent side is 5 units, how can you find the hypotenuse?

What is the first step in solving a right triangle?

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