Trigonometry

🔺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.

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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=c2a^2 + b^2 = c^2) 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 2π2\pi radians
  • To convert from degrees to radians, multiply by π180\frac{\pi}{180}
    • For example, 45° = 45π180=π4\frac{45\pi}{180} = \frac{\pi}{4} radians
  • To convert from radians to degrees, multiply by 180π\frac{180}{\pi}
    • For example, π6\frac{\pi}{6} radians = π6×180π=30\frac{\pi}{6} \times \frac{180}{\pi} = 30°
  • Acute angles are between 0° and 90° (or 0 and π2\frac{\pi}{2} radians)
  • Complementary angles add up to 90° (or π2\frac{\pi}{2} radians)
  • Supplementary angles add up to 180° (or π\pi 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 cc)
  • The other two sides are called legs or catheti (usually denoted as aa and bb)
  • The Pythagorean theorem states that in a right triangle, a2+b2=c2a^2 + b^2 = c^2
    • For example, if a=3a = 3 and b=4b = 4, then c=32+42=5c = \sqrt{3^2 + 4^2} = 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=oppositehypotenuse\sin \theta = \frac{\text{opposite}}{\text{hypotenuse}}
  • Cosine (cos) of an angle is the ratio of the adjacent side to the hypotenuse: cosθ=adjacenthypotenuse\cos \theta = \frac{\text{adjacent}}{\text{hypotenuse}}
  • Tangent (tan) of an angle is the ratio of the opposite side to the adjacent side: tanθ=oppositeadjacent\tan \theta = \frac{\text{opposite}}{\text{adjacent}}
  • Reciprocal trigonometric ratios include cosecant (csc), secant (sec), and cotangent (cot)
    • cscθ=1sinθ\csc \theta = \frac{1}{\sin \theta}, secθ=1cosθ\sec \theta = \frac{1}{\cos \theta}, cotθ=1tanθ\cot \theta = \frac{1}{\tan \theta}
  • 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=5a = 5 and c=13c = 13, then b=13252=12b = \sqrt{13^2 - 5^2} = 12
  • Use trigonometric ratios (SOHCAHTOA) when given one side length and one angle measure
    • For example, if a=8a = 8 and θ=30°\theta = 30°, then c=acosθ=8cos30°9.24c = \frac{a}{\cos \theta} = \frac{8}{\cos 30°} \approx 9.24
  • Inverse trigonometric functions (arcsin, arccos, arctan) help find angles when given side lengths
    • For example, if a=4a = 4 and c=8c = 8, then θ=arcsinac=arcsin48=30°\theta = \arcsin \frac{a}{c} = \arcsin \frac{4}{8} = 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

Common Mistakes and How to Avoid Them

  • Confusing sine, cosine, and tangent ratios
    • Remember SOHCAHTOA: Sine = Opposite/Hypotenuse, Cosine = Adjacent/Hypotenuse, Tangent = Opposite/Adjacent
  • 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=c2a^2 + b^2 = c^2, not a+b=ca + 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

  1. 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 xx and the adjacent side be yy
      • sin35°=x20\sin 35° = \frac{x}{20}, so x=20sin35°11.47x = 20 \sin 35° \approx 11.47 units
      • cos35°=y20\cos 35° = \frac{y}{20}, so y=20cos35°16.35y = 20 \cos 35° \approx 16.35 units
  2. 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\theta
      • tanθ=oppositeadjacent=125\tan \theta = \frac{\text{opposite}}{\text{adjacent}} = \frac{12}{5}
      • θ=arctan12567.38°\theta = \arctan \frac{12}{5} \approx 67.38°
  3. 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\theta
      • tanθ=riserun=112\tan \theta = \frac{\text{rise}}{\text{run}} = \frac{1}{12}
      • θ=arctan1124.76°\theta = \arctan \frac{1}{12} \approx 4.76°
  4. 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\sqrt{3} : 2
      • If the shortest side (opposite to 30°) is 6 units, then:
        • The hypotenuse (opposite to 90°) is 6×2=126 \times 2 = 12 units
        • The side opposite to 60° is 6×310.396 \times \sqrt{3} \approx 10.39 units
  5. Prove that in a 45-45-90 triangle, the length of the hypotenuse is 2\sqrt{2} times the length of a leg.

    • Solution:
      • Let the length of each leg be xx
      • By the Pythagorean theorem, x2+x2=c2x^2 + x^2 = c^2, where cc is the hypotenuse
      • Simplifying, 2x2=c22x^2 = c^2
      • Taking the square root of both sides, 2x=c\sqrt{2}x = c
      • Therefore, the hypotenuse is 2\sqrt{2} times the length of a leg


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.