Philippa M. Rhodes

Write-up 1

Examine graphs of

y = a sin (bx + c)

for different values of a, b, c.

First, let b = 2 and c = 1, and let's look at the graph of y = a sin (2x + 1) when a = 1.


Now, overlay the graph when a = 2.


Notice that the amplitude has changed to 2.

What happens when a is negative? Let's compare the graphs when a = 2 and a = -2.


The amplitude is still 2, but the graph is flipped.


Here are the graphs when a = 0.5 and when a = -0.5.

 

Once again, the amplitude has been changed to a. When a is negative, the graph is flipped. Thus, we see that the value of a determines the amplitude and the concavity of the graph of y = a sin (bx + c).

For the next example, let a = 3 and c = -1, and vary the value of b. First, let b = 1.


Now, overlay the graph when b = 2.

 

Notice that within one period of the graph when b = 1, there are two periods when b = 2.

Let b = -2,

 


The graph still has two cycles within b = 1 (not shown), but it has been shifted to the right.

Let b = 0.5 (the green curve),

 


there are two periods of the graph when b = 1 contained in one period of the graph when b = 0.5. In other words, y = 3 sin (0.5 x -1) cycles one-half (0.5) time within y = 3 sin (x -1).

Here is the graph when b = -0.5.

 


Notice that the negative value for b has again shifted the graph to the right.

 

Lastly, let a = 1 and let b = 1. So,

We are familiar with the graph when c = 0.


Let's see what happens when c = 1 (red), 2 (green), and 3 (blue).

Notice that the graph shifts to the left and crosses the y-axis when y = - c.

What happens when c is negative? Let c = -1 (dull green), -2 (red), and -3 (bright green).


Again, the graph crosses the y-axis when y = - c.

Here, c = 0.5 (red) and c = - 0.5 (green).


Thus, varying the value of c shifts the graph to the left and right and the y-intercept is - c.

By looking at y = 18 sin (-7x + 5.4) we know that the amplitude is a = 18, there are 7 ...



Return to Philippa's EMT Homepage