Phase Shifts: Videos & Practice Problems

Understanding the transformations of sine and cosine functions is essential in trigonometry, particularly when it comes to phase shifts. A phase shift refers to a horizontal shift of the graph, which can occur to the left or right, similar to how a vertical shift moves the graph up or down.

To illustrate a phase shift, consider the cosine function, cos(x), which typically starts at a maximum value of 1 and follows a wave-like pattern. When a number is added or subtracted inside the cosine function, such as in cos(x - h), it results in a phase shift. For example, the function cos(x - \frac{\pi}{2}) shifts the graph to the right by \(\frac{\pi}{2}\) units. This can be confirmed by evaluating specific points:

• cos(0 - \frac{\pi}{2}) = cos(-\frac{\pi}{2}) = 0
• cos(\frac{\pi}{2} - \frac{\pi}{2}) = cos(0) = 1
• cos(\pi - \frac{\pi}{2}) = cos(\frac{\pi}{2}) = 0
• cos(\frac{3\pi}{2} - \frac{\pi}{2}) = cos(\pi) = -1
• cos(2\pi - \frac{\pi}{2}) = cos(\frac{3\pi}{2}) = 0

By connecting these points, it becomes evident that the entire cosine wave has been shifted to the right, demonstrating the effect of the phase shift.

To determine the direction and magnitude of the phase shift, examine the expression inside the trigonometric function. If the function is in the form cos(bx - h), the graph shifts to the right by \(\frac{h}{b}\) units. Conversely, if it is in the form cos(bx + h), the graph shifts to the left by \(\frac{h}{b}\) units. For instance, in the function cos(x - \frac{\pi}{2}), the value of h is \(\frac{\pi}{2}\) and b is 1, resulting in a right shift of \(\frac{\pi}{2}\) units.

Interestingly, phase shifts can also make a cosine graph resemble a sine graph. For example, the function y = sin(2x + \frac{\pi}{2}) can be analyzed by first determining its period, which is given by the formula Period = \frac{2\pi}{b}. Here, b is 2, leading to a period of \(\pi\). The phase shift can be calculated as \(\frac{\frac{\pi}{2}}{2} = \frac{\pi}{4}\), indicating a left shift of \(\frac{\pi}{4}\) units.

In summary, phase shifts are a fundamental concept in understanding the behavior of sine and cosine functions. By recognizing how changes within the function affect the graph's position, students can effectively analyze and graph these trigonometric functions.

To graph the function \( y = 3 \sin(x + \pi) \), we start by identifying the general form of the sine function, which is expressed as \( y = a \sin(b(x - h)) + k \). In this case, we can see that the amplitude \( a \) is 3, indicating that the graph will oscillate between 3 and -3 on the y-axis. The absence of a vertical shift means \( k = 0 \).

Next, we analyze the term \( x + \pi \). To fit this into the general form, we rewrite it as \( x - (-\pi) \). This reveals that \( h = -\pi \). The value of \( b \) is 1 since there is no coefficient in front of \( x \). The phase shift can be calculated using the formula \( \text{Phase Shift} = \frac{h}{b} \). Substituting our values gives us \( \frac{-\pi}{1} = -\pi \), indicating a shift of \( \pi \) units to the left.

Now, we can sketch the graph of \( y = 3 \sin(x) \) first. The standard sine function starts at the origin, reaches a peak of 3 at \( \frac{\pi}{2} \), crosses the x-axis at \( \pi \), and reaches a valley of -3 at \( \frac{3\pi}{2} \). This pattern continues indefinitely in both directions.

After establishing the basic sine graph, we apply the phase shift of \( \pi \) units to the left. This adjustment moves the peak from \( \frac{\pi}{2} \) to \( -\frac{\pi}{2} \), the x-intercept from \( \pi \) to \( -\pi \), and the valley from \( \frac{3\pi}{2} \) to \( -\frac{3\pi}{2} \). The graph will now start at the center line, peak at \( -\frac{\pi}{2} \), cross through \( -\pi \), and reach a valley at \( -\frac{3\pi}{2} \), continuing this oscillation to the left and right.

In summary, the graph of \( y = 3 \sin(x + \pi) \) exhibits an amplitude of 3, a phase shift of \( \pi \) units to the left, and follows the standard sine wave pattern, adjusted accordingly. This understanding of amplitude, phase shift, and the general sine function form is crucial for accurately graphing trigonometric functions.