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IB DP Maths AA SL Study Notes

3.4.1 Basic Equations

Understanding Intervals

Before delving into the equations, it's paramount to understand the concept of an 'interval'. In mathematics, an interval represents a set of numbers lying between two values. These values can either be inclusive, where they are part of the set, or exclusive, where they are not.

  • Closed Interval: This includes both endpoints. It's denoted as [a, b].
  • Open Interval: This excludes both endpoints. It's represented as (a, b).
  • Half-Open Interval: This includes one endpoint but not the other. It's denoted as [a, b) or (a, b].

When solving trigonometric equations, we often seek solutions within a specific interval, typically [0, 360°) or [0, 2π) for radian measures. These intervals cover the values in one complete cycle of the trigonometric functions. For more on understanding radians, see Introduction to Radians.

Solving Basic Trigonometric Equations

To solve trigonometric equations, one requires a clear understanding of the trigonometric functions and their properties. Here, we'll delve deeper into methods to solve basic equations and find solutions within given intervals.

Step-by-Step Approach

1. Identify the Function and Interval: Start by determining which trigonometric function you're dealing with (sine, cosine, tangent) and the interval within which you need to find the solution. For visual representations, check out Graphs of Sine.

2. Isolate the Trigonometric Function: If the equation is more complex, aim to isolate the trigonometric function on one side of the equation.

3. Find the Principal Solution: Solve the equation as you would a basic algebraic equation. The solution you obtain is termed the 'principal solution'.

4. Determine the Period of the Function: Each trigonometric function has a period after which it repeats its values. Knowing this period is instrumental in finding all possible solutions within the given interval.

5. Find All Solutions Within the Interval: Using the period, calculate all possible solutions within the given interval. Remember, trigonometric functions can have multiple valid solutions within any interval. Understanding concepts such as Double and Half Angle Identities can also be helpful.

Example Problem

Problem: Solve sin(x) = 0.5 for x in the interval [0, 360°).

Solution:

  • We are dealing with the sine function, and the interval is [0, 360°).
  • The sine function is already isolated.
  • The principal solution is found by considering the special angle whose sine is 0.5. We know sin(30°) = 0.5, so x = 30° is a principal solution.
  • The sine function has a period of 360°. This means that after every 360°, the sine values repeat.
  • To find all solutions, we consider the symmetry of the sine function. We know that sin(x) = sin(180° - x). So, another solution within the interval is 180° - 30° = 150°.

Therefore, the solutions for the equation within the given interval are 30° and 150°.

Deep Dive into Trigonometric Functions

To further understand how to solve basic trigonometric equations, it's essential to delve deeper into the properties and characteristics of the trigonometric functions.

Sine Function

The sine function, denoted as sin(x), is a periodic function with a period of 360° or 2π radians. It starts from 0, reaches a maximum of 1 at 90°, goes back to 0 at 180°, reaches a minimum of -1 at 270°, and returns to 0 at 360°. Explore how this relates to Arc Length for better understanding.

Key Characteristics:

  • Amplitude: The maximum value of the sine function is 1, and the minimum value is -1.
  • Period: The sine function repeats its values after every 360° or 2π radians.
  • Symmetry: The sine function is symmetrical about the y-axis.

Cosine Function

The cosine function, denoted as cos(x), is also a periodic function with a period of 360° or 2π radians. It starts from 1, goes to 0 at 90°, reaches a minimum of -1 at 180°, returns to 0 at 270°, and goes back to 1 at 360°.

Key Characteristics:

  • Amplitude: The maximum value of the cosine function is 1, and the minimum value is -1.
  • Period: The cosine function repeats its values after every 360° or 2π radians.
  • Symmetry: The cosine function is symmetrical about the origin.

Tangent Function

The tangent function, denoted as tan(x), is different from sine and cosine. It's the ratio of sine to cosine. The function has vertical asymptotes (values where the function is undefined) at odd multiples of 90°.

Key Characteristics:

  • Amplitude: The tangent function doesn't have a fixed amplitude. It can take any real value.
  • Period: The tangent function repeats its values after every 180° or π radians.
  • Symmetry: The tangent function is symmetrical about the origin.

Key Points to Remember

  • Symmetry in Trigonometric Functions: The sine and cosine functions are symmetrical, which often leads to multiple solutions within one period. Tangent, however, repeats after 180° and requires consideration of additional solutions based on its symmetry.
  • Principal Solution and Additional Solutions: The first solution you find (principal) is not the only one. Use the function's period to explore additional solutions within the interval.
  • Check the Solutions: Always verify your solutions by plugging them back into the original equation to ensure they satisfy the equation. For further practice, look into solving related problems such as Exponential Equations.

FAQ

The number of solutions a trigonometric equation has within a given interval depends on the function and the nature of the equation. For instance, within the interval [0, 360°), the sine and cosine functions can have at most two solutions, while the tangent function can have multiple solutions due to its shorter period of 180°. To determine the exact number of solutions, one should graphically analyse the function or use algebraic methods. It's also essential to consider the domain restrictions of the function, especially for functions like tangent, which are undefined at certain points.

The principal solution is the smallest positive angle that satisfies the trigonometric equation. It serves as a reference point from which other solutions can be derived, especially given the periodic nature of trigonometric functions. Once the principal solution is identified, other solutions can be found by adding or subtracting the function's period. For trigonometric equations, the principal solution often lies within the interval [0, 360°) or [0, 2π) for radian measures. It provides a starting point for understanding the equation's behaviour and finding all possible solutions.

Absolutely! Trigonometric equations play a crucial role in various real-world scenarios. For instance, in physics, they are used to model wave behaviours, such as sound waves or light waves. In engineering, trigonometric equations can help determine the angles and lengths needed in construction projects. In navigation, they are used to calculate distances and directions. In electronics, they help in analysing alternating current circuits. The ability to solve these equations allows professionals in these fields to make accurate predictions, designs, and analyses, making them indispensable in many scientific and engineering applications.

The unit circle is a circle of radius 1 centred at the origin of a coordinate plane. It's a powerful tool for visualising trigonometric functions and their values. Each point on the unit circle corresponds to an angle, and the coordinates of that point give the cosine and sine values for that angle. By examining the unit circle, one can quickly determine the sine, cosine, and tangent values for various angles, making it easier to solve trigonometric equations. Moreover, the unit circle provides a geometric interpretation of the periodic nature of these functions, as moving around the circle corresponds to adding or subtracting the function's period.

The interval [0, 360°) is commonly used because it represents one complete cycle of the trigonometric functions sine, cosine, and tangent. Within this interval, each of these functions goes through all its possible values once. By examining solutions within this interval, we can understand the basic behaviour of the function. Once we have the solutions in this primary interval, we can use the periodic nature of trigonometric functions to find solutions in other intervals. It's a standard convention that provides a consistent framework for understanding and solving trigonometric equations.

Practice Questions

Solve for x in the equation cos(x) = 0.7 for x in the interval [0, 360°).

To solve the equation cos(x) = 0.7 within the interval [0, 360°), we first find the principal solution using the inverse cosine function. This gives us: x = inverse cos(0.7) However, the cosine function is symmetrical about the y-axis, so we have another solution in the interval [0, 360°), which is: x = 360° - inverse cos(0.7) Thus, the two solutions for x in the given interval are x = inverse cos(0.7) and x = 360° - inverse cos(0.7).

Solve for x in the equation tan(x) = 1.5 for x in the interval [0, 360°).

To solve the equation tan(x) = 1.5 within the interval [0, 360°), we use the inverse tangent function to find the principal solution: x = inverse tan(1.5) The tangent function has a period of 180°. Therefore, the next solution within the interval [0, 360°) is: x = 180° + inverse tan(1.5) Thus, the two solutions for x in the given interval are x = inverse tan(1.5) and x = 180° + inverse tan(1.5).

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
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Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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