First Derivative

In calculus, the most common operation you will perform is taking the derivative of a number. Take the following equation as an example.

 

cubic_function

 

Let’s plug in some numbers into this function and then plot it to get some more information about its shape.

x 4x^{3} 5x^{2} 6x d + e - f + 10
1 4 5 6 13
2 16 20 24 22
3 36 45 54 37
4 64 80 96 58
5 100 125 150 85
6 144 180 216 118

 

Plotting this onto a graph, we get:

graph_example

When we plot the graph, we get important information such as:

  • Where the function increases and decreases
  • How fast it is increasing and decreasing at certain points

For example, at x equal to 5, we can see the function is increasing rapidly given the slope is quite steep at that point. While graphing gives us useful information, it is not practical to graph every equation in order to extract information from it.

Taking the derivative of a function solves this problem. The derivative of a function is a measure of how fast or slow the function is increasing or decreasing at any given point in the function. This is also known as the rate of change.

First Derivative Properties

The most important property of taking the derivative of a function is something known as continuity. Take a look at the examples below to get an idea of what being continuous means.

graph_example_sin2

graph_example_break

graph_example_jump

A Continuous No breaks in the line
B Not Continuous A break
C Not Continuous A hole

Critical Point

Take any random point on the graph of a continuous function as an example. If the first derivative at that point is zero, that point is a critical point of the function. In formal terms:

first_derivative_critical_point

Let’s see what this translates into. We know that the first derivative is the rate of change of a function. If the first derivative is equal to zero, this is like saying there is no change at that point.

graph_example_linear_equation

As you can see, there are two points on the graph where the rate of change appears to be zero. We can see this by drawing the slopes at each point. At points A and C, the slope looks flat. Compare this to point B, where the slope is diagonal.

Rolle’s Theorem

Rolle’s theorem is an important theorem because it helps you understand the mean value theorem. Rolle’s theorem states the following:

Assumption 1 If you have a function that:
Sub-condition Is continuous on an interval [x,y]
Sub-condition Can be differentiated on the interval (x,y)
Sub-condition The function at x - f(x) - is equal to the function at y - f(y)
Assumption 2 There is a point z where f’(z) = 0
Result Then the function has a critical point in the interval (x,y)

Which is basically saying that between these two points there is a critical point if these conditions are satisfied.

Mean Value Theorem

The mean value theorem is similar to Rolle’s theorem, except that it actually needs Rolle’s theorem in order to prove that it exists. The MVT says the following.

Assumption 1 If you have a function that:
Sub-condition Is continuous on an interval [x,y]
Sub-condition Can be differentiated on the interval (x,y)
Result There is a point z where f’(z) is \frac{f(y) - f(x)}{y-x}

Problem 1

The owner of a shop wants to know more about the influence of the changes in the average price of the goods in their store on the demand. You have the following function:

linear_equation

This function details the price and demand of the store. In order to give the owner more information, you decide to tell them at which price the demand stops increasing or decreasing. In other words, you want to find the relative extrema at this point, if there are any. Solve this problem.

Problem 2

You are interested in the following function.

fourth_degree_equation

Conduct a first derivative test for local extrema. Then, plot the function onto a graph, marking the extrema.

 

Problem 3

You are given the following information.

Function f(x)
Point 1 Continuous on [4,10]
Point 2 Differentiated on [4,10]
Point 3 f(4) = -1
Point 4 f’(x) geq 8

Using the mean value theorem, what is the biggest value f(10) can take on?

Solution Problem 1

In order to find the point at which the price stops increasing or decreasing, we need to differentiate the equation and solve for where y will be equal to zero.

First, we need to apply the rule that deals with adding or subtracting two functions.

composite_function

This tells us that the first derivative of two functions added or subtracted from each other is the same as the sum or difference of the first derivative of each separate function.

derivative_rules

The first derivative of a constant is 0, so we are left with :

first_derivative_example

Which means that the demand doesn’t change when the price is zero.

Solution Problem 2

For this equation, we were asked to conduct a first derivative test to find local extrema. First, we need to take the first derivative of the equation.

first_derivative_example2 first_derivative_example3

Now, we simply see which value of y where x is equal to zero.

    \[ y = 2x^{3} + 4 \]

    \[ y = 2(0^{3}) + 4 \]

    \[ y = 0 + 4 = 4 \]

The point (0,4) is a candidate for local extrema.

Solution Problem 3

For this problem, we simply conduct the MVT backwards.

    \[ f(10) - f(4) = f'(z)(10 - 4) \]

    \[ f(10) = f'(z)(10 - 4) + f(4) \]

    \[ f(10) = 6(f'(z)) -1 \]

Since f’(z) geq 8, we know that the highest number that z can be is 8.

    \[ f(10) = 6(8) -1 = 47 \]

Need a Maths teacher?

Did you like the article?

1 Star2 Stars3 Stars4 Stars5 Stars 5.00/5 - 1 vote(s)
Loading...

Danica

Located in Prague and studying to become a Statistician, I enjoy reading, writing, and exploring new places.