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Increasing and Decreasing Functions

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It is important to be able to distinguish between when functions are increasing and when they are decreasing. In business this could mean the difference between making money and losing money. In physics it could mean the difference between speeding up and slowing down. 

How do you decide when a function is increasing or decreasing? 

Watch This

http://www.youtube.com/watch?v=78b4HOMVcKM James Sousa: Determine Where a Function is Increasing or Decreasing

Guidance

Increasing means places on the graph where the slope is positive. 

The formal definition of an increasing interval is: an open interval on the  x axis of (a, d)  where every b, c \in (a, d) with b < c has f(b) \le f(c) .

A interval is said to be strictly increasing if f(b) < f(c)  is substituted into the definition. 

Decreasing means places on the graph where the slope is negative. The formal definition of decreasing and strictly decreasing are identical to the definition of increasing with the inequality sign reversed. 

A function is called monotonic if the function only goes in one direction and never switches between increasing and decreasing.

Identifying analytically where functions are increasing and decreasing often requires Calculus. For PreCalculus, it will be sufficient to be able to identify intervals graphically and through your knowledge of what the parent functions look like. 

Example A

Identify which of the basic functions are monotonically increasing. 

Solution: Of the basic functions, the monotonically increasing functions are:

f(x) = x, f(x) = x^3, f(x) = \sqrt{x}, f(x) = e^x, f(x) = \ln x, f(x) = \frac{1}{1 + e^{-x}}

The only basic functions that are not monotonically increasing are:

f(x) = x^2, f(x) = |x|, f(x) = \frac{1}{x}, f(x) = \sin x

Example B

Identify whether the green, red or blue function is monotonically increasing and explain why. 

Solution: The green function seems to be discrete values along the line y = x + 2 . While the discrete values clearly increase and the line would be monotonically increasing, these values are missing a key part of what it means to be monotonic. The green function does not have a positive slope and is therefore not monotonic. 

The red function also seems to be increasing, but the slope at every  x value is zero. In Calculus the definition of monotonic will be refined to handle special cases like this. For now, this function is not monotonic.

The blue function seems to be y = \sqrt{x} - 2 , is increasing everywhere that is visible, and probably extends to the right. This function is monotonic where the function is defined for x \in (0, \infty)

Example C

Estimate the intervals where the function is increasing and decreasing. 

Solution:

Increasing: x \in (- \infty, -4) \cup (-2, 1.5)

Decreasing:  x \in (-4, -2) \cup (1.5, \infty)

Note that open intervals are used because at x = -4, -2, 1.5  the slope of the function is zero. This is where the slope transitions from being positive to negative. The reason why open parentheses are used is because the function is not actually increasing or decreasing at those specific points. 

Concept Problem Revisited

Increasing is where the function has a positive slope and decreasing is where the function has a negative slope. A common misconception is to look at the squaring function and see two curves that symmetrically increase away from zero. Instead, you should always read functions from left to right and draw slope lines and decide if they are positive or negative. 

Vocabulary

Increasing over an interval means to have a positive slope over that interval. 

Decreasing over an interval means to have a negative slope over that interval. 

Monotonic means that the function doesn’t switch between increasing and decreasing at any point. 

Strictly is an adjective that alters increasing and decreasing to exclude any flatness. 

Guided Practice

1. Estimate where the following function is increasing and decreasing. 

2. Estimate where the following function is increasing and decreasing. 

3. A continuous function has a global maximum at the point (3, 2), a global minimum at (5, -12) and has no relative extrema or other places with a slope of zero. What are the increasing and decreasing intervals for this function? 

Answers:

1. Increasing: x \in (-\infty, -1.5) \cup (1.5, \infty) . Decreasing:  x \in (-1.5, 1.5)

2. Increasing x \in (-\infty, -4) \cup (-4, -2.7) \cup (-1, 2) \cup (2, \infty) . Decreasing x \in (-2.7, -1)

3. Increasing x \in (-\infty, 3) \cup (5, \infty) . Decreasing x \in (3, 5)

Notice that the  y coordinates are not used in the intervals. A common mistake is to want to use the  y coordinates. 

Practice

Use the graph below for 1-2.

1. Identify the intervals (if any) where the function is increasing.

2. Identify the intervals (if any) where the function is decreasing.

Use the graph below for 3-4.

3. Identify the intervals (if any) where the function is increasing.

4. Identify the intervals (if any) where the function is decreasing.

Use the graph below for 5-6.

5. Identify the intervals (if any) where the function is increasing.

6. Identify the intervals (if any) where the function is decreasing.

Use the graph below for 7-8.

7. Identify the intervals (if any) where the function is increasing.

8. Identify the intervals (if any) where the function is decreasing.

Use the graph below for 9-10.

9. Identify the intervals (if any) where the function is increasing.

10. Identify the intervals (if any) where the function is decreasing.

11. Give an example of a monotonically increasing function.

12. Give an example of a monotonically decreasing function.

13. A continuous function has a global maximum at the point (1, 4), a global minimum at (3, -6) and has no relative extrema or other places with a slope of zero. What are the increasing and decreasing intervals for this function? 

14. A continuous function has a global maximum at the point (1, 1) and has no other extrema or places with a slope of zero. What are the increasing and decreasing intervals for this function? 

15. A continuous function has a global minimum at the point (5, -15) and has no other extrema or places with a slope of zero. What are the increasing and decreasing intervals for this function?

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