Consider the graph of
given in Figure 2.10. Since the parabola opens
downward with the vertex at (2, 9), the function
must increase until it reaches this peak at , and
decrease thereafter. Notationally we’ll write this as
for and for
Using the interval shown below the figure,
we see that any larger input value from the interval
will indeed produce a larger output value, and
on the interval. For instance,
and and
A calculator check is shown in the figure. Note the outputs are increasing until ,
then they begin decreasing.
EXAMPLE 5
䊳
Finding Intervals Where a Function Is Increasing
or Decreasing
Use the graph of v(x) given to name the interval(s)
where v is increasing, decreasing, or constant.
Solution
䊳
From left to right, the graph of v increases until
leveling off at ( , 2), then it remains constant
until reaching (1, 2). The graph then increases
once again until reaching a peak at (3, 5) and
decreases thereafter. The result is for
for and
v(x) is constant for .
Now try Exercises 33 through 36
䊳
Notice the graph of f in Figure 2.10 and the graph of v in Example 5 have some-
thing in common. It appears that both the far left and far right branches of each graph
point downward (in the negative y-direction). We say that the end-behavior of both
graphs is identical, which is the term used to describe what happens to a graph as be-
comes very large. For , we say a graph is, “up on the right” or “down on the
right,” depending on the direction the “end” is pointing. For , we say the graph
is “up on the left” or “down on the left,” as the case may be.
EXAMPLE 6
䊳
Describing the End-Behavior of a Graph
The graph of is shown. Use the
graph to name intervals where f is increasing or
decreasing, and comment on the end-behavior of
the graph.
Solution
䊳
From the graph we observe that for
, and for .
The end-behavior of the graph is down on the left,
and up on the right (down/up).
Now try Exercises 37 through 40
䊳
x 11, 12f 1x2Tx 1q, 12 ´ 11, q2
f
1x2c
f
1x2 x
3
3x
x 6 0
x 7 0
冟
x
冟
x 12, 12
x 13, q2,x 1q, 22 ´ 11, 32, v1x2T
v1x2c
2
x 2
8 7 7
f 1x
2
27 f 1x
1
2f 1127 f 122
x
2
7 x
1
1 7 2
f
1x2c
13, 22
x 12, q2.f
1x2Tx 1q, 22f 1x2c
x 2
f
1x2x
2
4x 5
2–7 Section 2.1 Analyzing the Graph of a Function 193
College Algebra Graphs & Models—
(5, 0)
(2, 9)
(0, 5)
(1, 0)
x (3, 2)
55
10
10
x
y
f(x) x
2
4x 5
Figure 2.10
WORTHY OF NOTE
Questions about the behavior of a
function are asked with respect to
the y outputs: is the function
positive, is the function increasing,
etc. Due to the input/ output,
cause/effect nature of functions,
the response is given in terms of x,
that is, what is causing outputs to
be positive, or to be increasing.
C. You’ve just seen how
we can determine where a
function is increasing or
decreasing
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