Increasing and Decreasing Functions
A function is "increasing" if the y-value increases as the x-value increases, like this:
It is easy to see that y=f(x) tends to go up as it goes along.
What about that flat bit near the start? Is that OK?
- Yes, it is OK if you say the function is Increasing
- But it is not OK if you say the function is Strictly Increasing (no flatness allowed)
What if you can't plot the graph to see if it is increasing? In that case is is good to have a definition using algebra.
For a function y=f(x):
|when x1 < x2 then f(x1) ≤ f(x2)||Increasing|
|when x1 < x2 then f(x1) < f(x2)||Strictly Increasing|
That has to be true for any x1, x2, not just some nice ones you choose.
The important parts are the < and ≤ signs ... remember where they go!
|This is also an increasing function
even though the rate of increase reduces
For An Interval
Usually you will only be interested in some interval, like this one:
This function is increasing for the interval shown
(it may be increasing or decreasing elsewhere)
The y-value decreases as the x-value increases:
For a function y=f(x):
|when x1 < x2 then f(x1) ≥ f(x2)||Decreasing|
|when x1 < x2 then f(x1) > f(x2)||Strictly Decreasing|
Notice that f(x1) is now larger than (or equal to) f(x2).
Let us try to find where a function is increasing or decreasing
Example: f(x) = x3-4x, for x in the interval [-1,2]
Let us plot it, including the interval [-1,2]:
Starting from -1 (the beginning of the interval [-1,2]):
Without exact analysis we cannot pinpoint where the curve turns from decreasing to increasing, so let us just say:
Within the interval [-1,2]:
- the curve decreases in the interval [-1, approx 1.2]
- the curve increases in the interval [approx 1.2, 2]
A Constant Function is a horizontal line:
In fact lines are either increasing, decreasing, or constant.
The equation of a line is:
y = mx + b
The slope m tells us if the function is increasing, decreasing or constant:
Strictly Increasing (and Strictly Decreasing) functions have a special property called "injective" or "one-to-one" which simply means you never get the same "y" value twice.
|General Function||"Injective" (one-to-one)|
Why is this useful? Because Injective Functions can be reversed!
You can go from a "y" value back to an "x" value (which you couldn't do if there were more than one possible "x" value).
Read Injective, Surjective and Bijective to find out more.