Fiche de révision : Understanding Right Triangle and Trigonometric Relationships

📋 Course Outline

1. Angle measure in right triangle
2. Sine of angle x
3. Expressions equal to 1/3
4. Cosine of x
5. Cosine of shifted angles

📖 1. Angle measure in right triangle

🔑 Key Concepts & Definitions

• The degree measure of an angle in a right triangle is denoted as x.
• In a right triangle, the measures of the two non-right angles sum to 90°.

📝 Essential Points

• The angle x is one of the non-right angles in a right triangle.
• Since the triangle is right-angled, the other non-right angle measures 90° - x.
• The sum of the two non-right angles always equals 90°.

💡 Key Takeaway

In a right triangle, the non-right angles are complementary, meaning their measures add up to 90°, with one angle measure denoted as x.

📖 2. Sine of angle x

🔑 Key Concepts & Definitions

• Sine of angle x: Defined as the ratio of the length of the side opposite to x to the hypotenuse in a right triangle.
  (Source: "Sine of angle x is defined as the ratio of the length of the side opposite to x to the hypotenuse.")

📝 Essential Points

• Given that sin x = 1/3, this establishes a specific ratio between the opposite side and the hypotenuse in the right triangle.
• The value of sin x relates directly to the triangle's side lengths, but it does not directly determine the cosine values of shifted angles.
• Expressions involving cosine of angles such as (x - 45°), (45° - x), (60° - x), and (90° - x) are not necessarily equal to 1/3 based solely on the sine value provided.

💡 Key Takeaway

The sine of angle x is the ratio of the opposite side to the hypotenuse, with sin x = 1/3 indicating this specific proportional relationship in the right triangle.

📖 3. Expressions equal to 1/3

🔑 Key Concepts & Definitions

• Expressions involving cosine and complementary angles can be equal to 1/3, based on the given sine value (sin x = 1/3).
• Using the relationship between sine and cosine, and the properties of complementary angles, certain cosine expressions can be equivalent to the sine value or related to it through identities.

📝 Essential Points

• When sin x = 1/3, the cosine of certain shifted angles (like (90° - x)) can also be equal to 1/3, depending on the angle relationships.
• Specifically, cos (90° - x) is equal to sin x, which is 1/3, making this expression also equal to 1/3.
• Other expressions involving cos (x - 45°), cos (45° - x), and cos (60° - x) are not necessarily equal to 1/3 based solely on the given sine value; their equality depends on specific angle relationships and identities.

💡 Key Takeaway

• The expression cos (90° - x) is equal to 1/3 because it is the cosine of the complementary angle to x, which equals sin x. Other expressions involving shifted angles require further evaluation to determine if they are also equal to 1/3.

📖 4. Cosine of x

🔑 Key Concepts & Definitions

• Cosine of angle x is the ratio of the adjacent side to the hypotenuse in a right triangle.
• In right triangles, cosine relates to the angle x and its trigonometric functions.

📝 Essential Points

• The cosine of x can be expressed in terms of the triangle's sides, specifically as the length of the side adjacent to x divided by the hypotenuse.
• The problem involves identifying which expressions involving cosine are also equal to 1/3, given that sin x = 1/3.
• The expressions to evaluate are: cos(x), cos(x - 45°), cos(45° - x), cos(60° - x), and cos(90° - x).
• The relationship between sine and cosine for complementary angles (see section 3) implies that cos(90° - x) is equal to sin x, which is 1/3.

💡 Key Takeaway

Cosine of x is the ratio of the adjacent side to the hypotenuse, and in right triangles, it directly relates to the angle x and its trigonometric functions, with specific expressions involving shifted angles potentially equal to 1/3.

📖 5. Cosine of shifted angles

🔑 Key Concepts & Definitions

• Cosine of shifted angles involves the cosine of angles such as (x - 45°), (45° - x), (60° - x), and (90° - x). These expressions relate to cosine identities and angle transformations, which help in simplifying and evaluating cosine functions of angles that are shifted by specific degrees.

📝 Essential Points

• The cosine of shifted angles is used to find expressions that are equal to a given value, such as 1/3 in this context.
• The angles (x - 45°), (45° - x), (60° - x), and (90° - x) are related through cosine identities and transformations.
• These shifted angles are useful in solving problems involving cosine functions where the angle is modified by a constant degree shift.
• The relationships between these angles and their cosines are essential for understanding how cosine functions behave under angle transformations.

💡 Key Takeaway

Cosine of shifted angles involves understanding how cosine functions change when the angle is shifted by specific degrees, which is crucial for simplifying and solving trigonometric expressions involving such shifts.

📊 Synthesis Tables

⚠️ Common Pitfalls & Confusions

1. Assuming cos x = sin x when x is complementary; only true for specific angles (e.g., 45°).
2. Believing all shifted cosine expressions (x - 45°, 60° - x) are equal to 1/3 without verification.
3. Confusing the relationship between sine and cosine of shifted angles; forgetting cos(90° - x) = sin x.
4. Mistaking the sum of non-right angles as always being 180°, ignoring the right angle.
5. Overgeneralizing that sin x = 1/3 implies cos x = 1/3; these are different ratios.
6. Not recognizing that the complement of x (90° - x) has a cosine equal to sin x.
7. Assuming the value of sin x directly determines the values of cos(x - 45°), cos(45° - x), etc., without using identities.

✅ Exam Checklist

• Know the definition of the angle measure in a right triangle and that the non-right angles are complementary (x and 90° - x).
• Understand that sin x = opposite / hypotenuse, with sin x = 1/3 indicating a specific ratio in the triangle.
• Recognize that cos(90° - x) = sin x, thus equals 1/3 when sin x = 1/3.
• Be able to identify which cosine expressions involving shifted angles (x - 45°, 45° - x, 60° - x, 90° - x) are equal to 1/3 based on identities.
• Master the relationship between sine and cosine for complementary angles.
• Know the basic cosine identities for shifted angles and how to apply them.
• Understand that cos x and sin x are different ratios and should not be confused.
• Be familiar with the properties of right triangles and the sum of their non-right angles.
• Recall that cosine of shifted angles involves transformations that can simplify or evaluate trigonometric expressions.
• Know SMITH's definition of the sine function and the properties of complementary angles.