putting functions together
Let with its natural or implied domain.
Graph of
The graph of has a “V”-shape, opening up, with a corner at .
This illustrates that decreases on and increases on . has a minimum value of ,
which occurs at .
If we think of tracing the domain from left to right on the graph, the corresponding
values of decrease from down to , then reverse direction and increase from back to .
The only values that can come out of the composition, , are the values of . Therefore, the values of are inside the interval .
Graph of
Since is a quadratic function, its graph has the shape of a parabola, opening down.
increases on and decreases on . has a maximum value of , which occurs at .
If we think of tracing the domain from left to right on the graph, the values of increase from up to , then reverse direction and decrease from back to .
The output from the function will become the input to . Therefore, in the
composition, the new input to will be the interval , which is the output of . This
interval is traced twice: the values going into will begin at , go up to , then turn
around and go back down to . The values of will similarly repeat themselves in
reverse.
Now to examine the composition:
First, the inside function:
The natural domain of is the whole real line. As we move from left to right along the real line, the domain numbers increase from to .
The corresponding movement in the range has the function values increasing from to . The maximum function value occurs at in the domain. As we keep moving beyond in the domain, the corresponding movement in the range is for the function values to decrease from to .
This path in the range of becomes the path in the domain of .
Movement inside the domain of .
Inside the natural domain of , the domain numbers will increase from to . Then they
will decrease from back down to .
Following our graph of , we will move from the far left toward the right until we reach
. Then we will turn around and move toward the left.
Put those together.
We have the input to moving from up to and then back down to . What are the corresponding function values for ?
- As the domain numbers move through , the values of decrease from to
and then increase from to . In the graph, we can see this switch as a
corner in the graph. The graph comes down to the right, hits the corner,
then moves back up to the right.
So, we really should view as and highlight the change in behavior.
- On , decreases from to .
- On , increases from to
That was on our first pass through .
The input to continues beginning at and moving towards .
As the overall domain to the whole compostion continues to cover , The function reverses this interval and sends the reversed interval into . As the overall composition domain continues to move through , the input to traces backwards from to . The overall composition graph keeps moving to the right, but it is retracing the left side in reverse, mirroring it.
- Moving backwards through , The values of first decrease from to , arriving
again at the corner that occurs at . Again, we really should look at as
.
- As the domain moves from to , decreases from to .
- As the domain moves from to , increases from to .
Since we reverse our travelling direction inside the domain, we hit the corner twice. Plus, right at our reversal, we will create a hill in the graph made by our own retracing of the domain steps.
All together now...
Graph of
Notice:
The function values of are all inside the interval , which was the range of . Thus, the
composition has no zeros, because has no zeros.
The graph of has two corners, because the corner of the graph of was included twice.
The corner in the graph of occurs when the input to is . When is the output of equal to ?
There is a corner at and one at .
The graph of has a hill at , the function values reverse themselves. Therefore, the composition also has its values reversing at .
is an axis of symmetry, because the input to (which is the output of ) reverses itself.
The formula for the composition looks like
This is includes an absolute value. We can remove the absolute value symbol by using
a piecewise defined formula.
We need to know where the inside of the absolute value bars equals .
These are the same places as the corners in the graph.
The inside is negative on and on . On these intervals, the formula will be
On the interval , the formula is
The middle piece is a quadratic with a negative leading coefficient. Its graph would be a parabola opening down, which is what we see in the graph.
Let with its natural or implied domain.
Let with its natural or implied domain.
Graph
Graph of
If we think of moving through the domain from to , then the graph illustrates that
will increase on and decrease on . The function has a maximum value of , which
occurs at .
However, the output from is not , so we will not get the full output of . In addition,
the output of will oscillate, which means the input values coming into will osciallate.
This will make the output of oscillate.
Graph of
The input into is the whole real line. We naturally think of moving left to right, from to . As we do this, the outputs from oscillate between and . Therefore, the inputs into oscillate back and forth inside the interval .
The output of keeps going back and forth across the interval .
This becomes the input into .
Therefore, the input into keep going back and forth across the interval .
What part of the graph of does this oscillation run across?
The only part of the graph of that is used is on the interval .
Therefore, that part of the graph of just keeps repeating back and forth.
Except, the “” in will become the input for . This will cause a horizontal shift for .
The smooth rounding of the sine curve will round out the corners of the absolute value graph and the period of will force the same period for the composition.
Notice:
We are repeating the piece of on the interval .
The minimum value of occurs at . When does the oputput of equal ?
when , which is when where . We can see this as the lower valleys in the graph of the composition.
The maximum value of occurs at . When does the oputput of equal ?
when , which is when and where . We can see this as the two peaks in the graph of the composition.
Graphically, we have to keep our eyes on several things at once. We watch the original input into , then we watch the output of and picture that as the new input into , then we watch the output coming from .
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more examples can be found by following this link
More Examples of Composition