6.1 Graphs of the sine and cosine functions  (Page 2/13)

[link] shows that the cosine function is symmetric about the y -axis. Again, we determined that the cosine function is an even function. Now we can see from the graph that $\cos (-x)=\cos x.$

Investigating sinusoidal functions

As we can see, sine and cosine functions have a regular period and range. If we watch ocean waves or ripples on a pond, we will see that they resemble the sine or cosine functions. However, they are not necessarily identical. Some are taller or longer than others. A function that has the same general shape as a sine or cosine function    is known as a sinusoidal function    . The general forms of sinusoidal functions are

Determining the period of sinusoidal functions

Looking at the forms of sinusoidal functions, we can see that they are transformations of the sine and cosine functions. We can use what we know about transformations to determine the period.

In the general formula, $B$ is related to the period by $P=\frac{2\pi }{\left|B\right|}.$ If $\left|B\right|>1,$ then the period is less than $2\pi$ and the function undergoes a horizontal compression, whereas if $\left|B\right|<1,$ then the period is greater than $2\pi$ and the function undergoes a horizontal stretch. For example, $f(x)=\sin (x),$ $B=\mathrm{1,}$ so the period is $2\pi ,$ which we knew. If $f(x)=\sin \left(2x\right),$ then $B=\mathrm{2,}$ so the period is $\pi$ and the graph is compressed. If $f(x)=\sin \left(\frac{x}{2}\right),$ then $B=\frac{1}{2},$ so the period is $4\pi$ and the graph is stretched. Notice in [link] how the period is indirectly related to $\left|B\right|.$

Determining amplitude

Returning to the general formula for a sinusoidal function, we have analyzed how the variable $B$ relates to the period. Now let’s turn to the variable $A$ so we can analyze how it is related to the amplitude , or greatest distance from rest. $A$ represents the vertical stretch factor, and its absolute value $\left|A\right|$ is the amplitude. The local maxima will be a distance $\left|A\right|$ above the vertical midline of the graph, which is the line $x=D;$ because $D=0$ in this case, the midline is the x -axis. The local minima will be the same distance below the midline. If $\left|A\right|>1,$ the function is stretched. For example, the amplitude of $f(x)=4\sin x$ is twice the amplitude of $$f(x)=2\sin x.$$ If $\left|A\right|<1,$ the function is compressed. [link] compares several sine functions with different amplitudes.