# Explainer: Definition of the Derivative

In this explainer, we will learn how to calculate the derivative of a function using the formal definition of the derivative as a limit.

Rates of change form a central concept in many real-world problems from mechanics to reaction rate and from modeling population growth to modeling radioactive growth and decay. In fact, the development of Newtonian mechanics is intimately connected to the mathematical study of rates of change. In many ways, one of the large contributing factors to the development of the study of rates of change was to be able to mathematically describe the mechanics of the real world.

If we consider a car accelerating from a position of rest, we see that the speed increases. Initially, we have a speed of zero; then, we might be going 10 miles per hour and then 20. However, what does it mean to say that we are traveling at a particular speed? For example, when you see that that speedometer reads 10 miles per hour, what does this mean? We might say that if we did not change our speed, we would travel 10 miles in an hour. Or we might consider how many meters we travel in a second, or maybe a smaller interval of time. However, clearly, if we consider how far we travel in zero time, it would be zero meters. In addition to all of this, we are not traveling with constant speed; we are accelerating.

The issue of trying to make these things mathematically rigorous is the challenge that the great mathematicians of the past such as Newton and Leibniz formalized for us as derivatives. In this explainer, we will introduce the definition of a derivative and how this enables us to deal with the apparent contradiction of talking about an instantaneous rate of change.

Conceptually, we can think of the idea of a derivative at a point as taking better and better approximations to the tangent of a curve at a given point. We can do this by considering the slopes of secant lines going and a second point on the curve as this point approaches .

Clearly, as gets closer to 0, the approximation to the slope of the tangent gets better and better. The derivative is formally defined as the limit of this process.

### Definition of the Derivative

The derivative of a function at a point is defined as where this limit exists. An alternative but equivalent definition of the derivative at is if the limit exists.

There are two common ways we denote derivatives: Leibniz’s notation and prime notation (sometimes referred to as Lagrange’s notation). For a function , Leibniz’s notation for the derivative is written using the infinitesimals and as which we read as “the derivative of with respect to ” or “ by .

Using prime notation, the derivative of with respect to is denoted which we read as “ prime of .

It is important for the student of calculus to be comfortable using either of the definitions of the derivative and with both forms of notation.

Often, using the first definition of the derivative is easier; however, in some cases it is just as simple to use either form. In the first example, we will demonstrate using the second form of the definition of the derivative.

### Example 1: Calculating the Derivative at a Point

Find the derivative of at the point from first principles.

Recall the definition of the derivative at :

Letting and , we have

Notice that the numerator is the difference of two squares that we can factor as follows:

Since , we can cancel this common factor in the numerator and denominator to get

We can now take the limit to get

Earlier, we gave the definition of the derivative at a point. However, we can actually consider the derivative as a function whose value is equal to the slope of the tangent to the curve at each value of . In the next example, we will demonstrate the process of finding the derivative as a function and using it to find the slope of the tangent.

### Example 2: Finding the Slope of the Tangent Using the Derivative

Let . Use the definition of the derivative to determine . What is the slope of the tangent to its graph at ?

We will use the definition of the derivative, to find an expression for . Using the function we have been given, we have

Expanding the parentheses in the numerator, we have

Since , we can cancel this common factor from the numerator and denominator to get

We can now take the limit as which gives us

Since the value of the derivative at a point is equal to the slope of the tangent to the curve at that point, the slope of the tangent to the graph at is equal to

Using the definition of the derivative, we can find the derivative of many different types of functions by using a number of algebraic techniques to evaluate the limits. In the next couple of examples, we will use the definition of the derivative to find the derivative of reciprocal and radical functions.

### Example 3: Finding the Derivative of a Reciprocal Function

Using the definition of a derivative, evaluate .

Recall the definition of the derivative:

Setting , we have

We begin by expressing this as a single fraction over a common denominator:

Since , we can cancel it from the numerator and denominator to get

We can now use the rules of finite limits to rewrite this as

Taking the limit as , we have

### Example 4: Finding the Derivative of a Radical Function

Find the derivative of the function using the definition of the derivative, and then state the domain of the function and the domain of its derivative.

Using the definition of the derivative that we have

Before we are able to evaluate this limit, we need to multiply the numerator and denominator by the conjugate of the numerator as follows:

Since , we can cancel it from the numerator and denominator to get

We can now use the rules of finite limits to rewrite this as

Taking the limit as , we get

We now need to consider the domain of the function and its derivative. Since the square root of a negative number is not a real number, the domain of should be restricted to the values of , which results in a nonnegative expression under the square root. Hence, the domain of is defined by

This is equivalent to which can be written as an interval as . Clearly, the same restriction applies to the derivative. However, we have the additional restriction that the denominator be nonzero. This occurs at the point where . Therefore, the domain of the derivative does not contain this point. Therefore, written in interval notation, the domain of is .

It is interesting to note that the derivative might only be defined on a subset of the domain of the original function. When we look at the differentiability of a function, we will explore this in more detail.

### Example 5: Evaluating Limits Using Derivatives

Evaluate .

When asked to evaluate a limit, often we need to relate it to other things we know. Here, we can see that it somehow looks similar to the definition of the derivative of :

We can therefore, try to rearrange the expression to see whether we can isolate the definition of the derivative of at a particular point. Therefore,

Since , we can see that the first part of this expression is the derivative . Hence, using the rules of limits, we have

Similarly, since , we can recognize this second expression as the derivative . Therefore,

When we gave the intuitive idea of the derivative, we relied on the fact that as we zoom in on a curve it increasingly resembles a straight line. However, this is not the case for all functions. In fact, there are many functions where this is not the case. In the next example, we will consider one such function.

### Example 6: Differentiability of a Function

Consider the function .

1. Find .
2. Find .
3. What can you conclude about the derivative of at ?

Part 1

We are considering the limit from the right as tends to zero. Since for all , we have

Part 2

Similarly, we can consider the limit from the left as tends to zero. Since for all , we have

Part 3

Since the right and left limits are unequal, the limit does not exist. We can, therefore, conclude that the derivative of does not exist at .

### Key Points

• The derivative of a function is defined as An alternative but equivalent definition of the derivative is There are two common ways we denote derivatives: Leibniz’s notation and prime notation (sometimes referred to as Lagrange’s notation). For a function , Leibniz’s notation for the derivative is written using the infinitesimals and as which we read as “the derivative of with respect to ” or “ by .
Using prime notation, the derivative of with respect to is denoted which we read as “ prime of .
• The derivative defines a function which is equal to the slope of the tangent at each point on the curve.
• A continuous function is not necessarily differentiable at all points in the domain of the function.