🔺Trigonometry Unit 2 – Acute Angles and Right Triangles
Acute angles and right triangles form the foundation of trigonometry. These concepts are crucial for understanding the relationships between angles and sides in triangles. From basic angle measurements to trigonometric ratios, this unit covers essential tools for solving problems in geometry and real-world applications.
The Pythagorean theorem and SOHCAHTOA mnemonic are key to mastering right triangle trigonometry. These principles, along with inverse trigonometric functions, enable us to solve for unknown sides and angles in right triangles, which has practical applications in fields like construction, navigation, and engineering.
we crunched the numbers and here's the most likely topics on your next test
Key Concepts
Acute angles measure less than 90 degrees
Right triangles contain one 90-degree angle and two acute angles
The side opposite the right angle is called the hypotenuse and is always the longest side
The sides adjacent to the right angle are called legs or catheti
Trigonometric ratios (sine, cosine, tangent) define relationships between the angles and sides of a right triangle
Pythagorean theorem (a2+b2=c2) relates the lengths of the three sides of a right triangle
SOHCAHTOA is a mnemonic for remembering the trigonometric ratios (Sine = Opposite/Hypotenuse, Cosine = Adjacent/Hypotenuse, Tangent = Opposite/Adjacent)
Inverse trigonometric functions (arcsin, arccos, arctan) help find angles when given side lengths
Angle Measurements
Angles are measured in degrees (°) or radians (rad)
One full rotation equals 360 degrees or 2π radians
To convert from degrees to radians, multiply by 180π
For example, 45° = 18045π=4π radians
To convert from radians to degrees, multiply by π180
For example, 6π radians = 6π×π180=30°
Acute angles are between 0° and 90° (or 0 and 2π radians)
Complementary angles add up to 90° (or 2π radians)
Supplementary angles add up to 180° (or π radians)
Properties of Right Triangles
Right triangles have one 90-degree angle
The side opposite the right angle is called the hypotenuse (usually denoted as c)
The other two sides are called legs or catheti (usually denoted as a and b)
The Pythagorean theorem states that in a right triangle, a2+b2=c2
For example, if a=3 and b=4, then c=32+42=5
The altitude (or height) of a right triangle is the perpendicular line segment from a vertex to the opposite side
The median of a right triangle is a line segment from a vertex to the midpoint of the opposite side
The angle bisector of a right triangle divides the opposite side into two segments proportional to the lengths of the other two sides
Trigonometric Ratios
Sine (sin) of an angle is the ratio of the opposite side to the hypotenuse: sinθ=hypotenuseopposite
Cosine (cos) of an angle is the ratio of the adjacent side to the hypotenuse: cosθ=hypotenuseadjacent
Tangent (tan) of an angle is the ratio of the opposite side to the adjacent side: tanθ=adjacentopposite
Reciprocal trigonometric ratios include cosecant (csc), secant (sec), and cotangent (cot)
cscθ=sinθ1, secθ=cosθ1, cotθ=tanθ1
Trigonometric ratios are constant for a given angle, regardless of the triangle's size
Special right triangles (30-60-90 and 45-45-90) have specific side length ratios that can be memorized
Solving Right Triangles
To solve a right triangle, find the unknown side lengths or angle measures using given information
Apply the Pythagorean theorem when given two side lengths to find the third side
For example, if a=5 and c=13, then b=132−52=12
Use trigonometric ratios (SOHCAHTOA) when given one side length and one angle measure
For example, if a=8 and θ=30°, then c=cosθa=cos30°8≈9.24
Inverse trigonometric functions (arcsin, arccos, arctan) help find angles when given side lengths
For example, if a=4 and c=8, then θ=arcsinca=arcsin84=30°
Always double-check your solutions by verifying they satisfy the Pythagorean theorem and trigonometric ratios
Applications in Real Life
Right triangles are used in various fields, such as architecture, engineering, and navigation
In construction, right triangles help determine roof pitches, stair angles, and building heights
For example, a 30° roof pitch means the roof rises 30 vertical units for every 100 horizontal units
Trigonometry is used in surveying to measure distances and angles between points
For instance, the height of a tree can be found by measuring the angle of elevation and the distance from the observer to the tree
Navigation relies on right triangles to calculate distances and directions
For example, a plane flying at a 40° angle to the ground for 100 miles will have traveled about 64 miles horizontally (100 × cos 40°)
Computer graphics and game development use trigonometry to rotate and transform objects in 2D and 3D space
Using the wrong angle or side when applying trigonometric ratios
Carefully identify the angle in question and its corresponding opposite, adjacent, and hypotenuse sides
Forgetting to square or take the square root when using the Pythagorean theorem
The Pythagorean theorem is a2+b2=c2, not a+b=c
Mixing up degrees and radians
Always check the mode of your calculator and convert angles if necessary
Rounding too early in multi-step problems
Carry extra decimal places throughout the calculation and round only the final answer
Not checking if the triangle is a right triangle before applying right triangle trigonometry
Verify that one of the angles is 90° or that the side lengths satisfy the Pythagorean theorem
Practice Problems and Solutions
In a right triangle, if one of the acute angles is 35° and the hypotenuse is 20 units, find the lengths of the other two sides.
Solution:
Let the opposite side be x and the adjacent side be y
sin35°=20x, so x=20sin35°≈11.47 units
cos35°=20y, so y=20cos35°≈16.35 units
A ladder 13 feet long leans against a wall. If the base of the ladder is 5 feet from the wall, find the angle the ladder makes with the ground.
Solution:
Let the angle be θ
tanθ=adjacentopposite=512
θ=arctan512≈67.38°
A ramp for wheelchairs must have a rise of no more than 1 unit for every 12 units of horizontal distance. What is the maximum angle the ramp can make with the ground?
Solution:
Let the angle be θ
tanθ=runrise=121
θ=arctan121≈4.76°
In a 30-60-90 triangle, if the shortest side is 6 units, find the lengths of the other two sides.
Solution:
In a 30-60-90 triangle, the sides are in the ratio of 1 : 3 : 2
If the shortest side (opposite to 30°) is 6 units, then:
The hypotenuse (opposite to 90°) is 6×2=12 units
The side opposite to 60° is 6×3≈10.39 units
Prove that in a 45-45-90 triangle, the length of the hypotenuse is 2 times the length of a leg.
Solution:
Let the length of each leg be x
By the Pythagorean theorem, x2+x2=c2, where c is the hypotenuse
Simplifying, 2x2=c2
Taking the square root of both sides, 2x=c
Therefore, the hypotenuse is 2 times the length of a leg