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IB DP Maths AA SL Study Notes

3.3.2 Graphs of Cosine

Introduction to the Cosine Graph

The cosine graph represents the x-coordinate of a point on the unit circle as it rotates about the origin. As the angle of rotation changes, the x-coordinate varies, producing the wave-like pattern of the cosine graph. This relationship between the unit circle and the cosine function provides a geometric interpretation of the cosine graph and its properties.

Key Features of the Cosine Graph

  • Amplitude: The amplitude of the cosine graph is the maximum distance from the x-axis, which is 1. This means the graph oscillates between -1 and 1. The amplitude represents the height of the wave.
  • Period: The period of the cosine graph is the interval in which the function completes one full cycle. For the standard cosine function, this period is 2π radians or 360°.
  • Frequency: The frequency of the cosine graph denotes the number of cycles the function completes in 2π radians. For the standard cosine graph, the frequency is 1.
  • Phase Shift: The standard cosine graph starts from the point (0,1), unlike the sine graph which starts from the origin. If the graph is shifted left or right, it undergoes a phase shift.
  • Vertical Shift: If the entire graph is moved up or down, it experiences a vertical shift.

Transformations of the Cosine Graph

Transformations can modify the appearance of the cosine graph. The general equation for a transformed cosine function is: y = a cos(bx + c) + d

Where:

  • a determines the amplitude. If a is negative, the graph reflects over the x-axis.
  • b affects the period. The period is recalculated as 2π/|b|.
  • c determines the phase shift. The graph shifts horizontally by c/b units.
  • d is the vertical shift, adjusting the graph d units vertically.

Detailed Analysis of Transformations

1. Vertical Stretching and Compression: The factor 'a' in the equation stretches or compresses the graph vertically. If |a| > 1, the graph stretches, and if 0 < |a| < 1, it compresses.

2. Horizontal Stretching and Compression: The factor 'b' influences the horizontal stretching or compression. A larger |b| value compresses the graph, while a smaller |b| value stretches it.

3. Reflections: If 'a' is negative, the graph reflects over the x-axis. Similarly, a negative 'b' reflects the graph over the y-axis.

4. Translations: The values of 'c' and 'd' translate or shift the graph. 'c' shifts the graph horizontally, while 'd' shifts it vertically.

Example 1: Graphing a Transformed Cosine Function

Consider the function y = 2 cos(x - π/3) + 3.

  • The amplitude is 2, indicating a vertical stretch.
  • The period remains 2π, as there's no coefficient with x.
  • The phase shift is π/3, shifting the graph to the right by π/3 units.
  • The vertical shift is 3, elevating the graph by 3 units.


To graph this function, start with the standard cosine curve. Then, apply the transformations in sequence: vertical stretch, horizontal shift, and vertical shift.

Example 2: Analysing a Complex Cosine Function

For the function y = -3 cos(2x + π/2) - 2:

  • The amplitude is 3, but the negative sign indicates a reflection over the x-axis.
  • The coefficient of x is 2, compressing the graph horizontally and giving a period of π.
  • The phase shift is -π/2, shifting the graph to the left by π/4 units.
  • The vertical shift is -2, moving the graph down by 2 units.

Real-World Applications of the Cosine Function

The cosine function plays a pivotal role in various real-world scenarios. In physics, it describes oscillations, such as pendulum motion. In engineering, it's crucial for understanding alternating current in electrical circuits. In computer graphics, it assists in rendering light reflections on surfaces. In astronomy, it aids in understanding the periodic motion of celestial bodies.

FAQ

When the cosine graph undergoes a horizontal reflection, it becomes the graph of the sine function. This is because reflecting the cosine graph about the y-axis results in the sine curve. Mathematically, this transformation can be represented as y = cos(-x), which is equivalent to y = sin(x). This property showcases the co-function relationship between sine and cosine.

To determine the number of cycles of a cosine graph within a specific interval, you need to consider the period of the function. Divide the length of the interval by the period of the cosine function. For instance, if you have a cosine function with a period of π and you want to know how many cycles it completes between 0 and 2π, you would calculate 2π/π = 2. This means the function completes two full cycles within the interval.

Both the sine and cosine functions are derived from the unit circle. The amplitude represents the maximum distance from the x-axis, which, for both functions, is the radius of the unit circle. Since the radius of the unit circle is 1, both the sine and cosine functions have an amplitude of 1. This is why they both oscillate between -1 and 1, regardless of their phase or other transformations.

In its standard form, the cosine function has an amplitude of 1. However, in real-world applications, the cosine function can be scaled to represent phenomena with larger or smaller oscillations. For example, in physics, when representing wave functions or alternating currents, a scaling factor (amplitude) other than 1 might be used to match real-world measurements. This scaling factor stretches or compresses the graph vertically, allowing the cosine function to exceed an amplitude of 1 or have an amplitude less than 1, depending on the context.

The primary difference between the starting points of the cosine and sine graphs is their y-values at x = 0. For the standard cosine function, cos(x), the graph starts at the point (0,1), meaning it begins at its maximum value. On the other hand, the standard sine function, sin(x), starts at the origin (0,0). This distinction is crucial when analysing phase shifts and determining the nature of a trigonometric function based on its graph.

Practice Questions

A cosine function has a period of π/2 and an amplitude of 5. The graph is reflected over the x-axis and shifted 3 units to the left. Write the equation of the function.

To determine the equation of the cosine function, let's consider the given properties:

  • The amplitude is 5, but since the graph is reflected over the x-axis, the coefficient of the cosine function will be -5.
  • The period is given as π/2. The standard period for a cosine function is 2π. The relationship between the standard period and the given period is 2π/b = π/2, which gives b = 4.
  • The graph is shifted 3 units to the left, which means the function will have a term (x + 3) inside the cosine function.

Using this information, the equation of the function is y = -5 cos(4x + 3).

The graph of a cosine function has a maximum value at x = π/4 and a minimum value at x = 5π/4. Determine the amplitude, period, and phase shift of the function.

For the given cosine function:

  • The maximum value occurs at x = π/4 and the minimum value at x = 5π/4. The difference between these two x-values is 5π/4 - π/4 = π, which is half the period of the cosine function. Therefore, the period is 2π.
  • The amplitude is half the distance between the maximum and minimum values of the function. Since the standard cosine function has a maximum value of 1 and a minimum value of -1, the amplitude is (1 - (-1))/2 = 1.
  • The phase shift can be determined by the x-value of the maximum point. Since the standard cosine function has its maximum at x = 0 and the given function has its maximum at x = π/4, the phase shift is π/4 units to the right.

Using this information, the amplitude is 1, the period is 2π, and the phase shift is π/4 to the right.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
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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