This section covers:

**Constant Rule****Power Rule****Product Rule****Quotient Rule****List of Rules****Examples of Constant, Power, Product and Quotient Rules****Derivatives of Trig Functions****Higher Order Derivatives****More Practice**

Note that you can use **www.wolframalpha.com** (or use app on smartphone) to check derivatives by typing in “derivative of x^2(x^2+1)”, for example. Really cool!

I promised you that I’d give you easier way to take derivatives, and the **constant**, **power**, **product,** **quotient** and **basic trigonometry function** rules make it much, much easier.

Let’s first talk about some **notation**. When we take the derivative, say of the function \(f\left( x \right)=x+3\), we typically say that we are taking the derivative of **\(y=f\left( x \right)\) with respect to x **or whatever the independent variable is (later we’ll take the derivative with respect to more than one variable). When we take the derivative with respect to

*x*, we may see it written the following ways: \({f}’\left( x \right),\,\,\,\,{y}’,\,\,\,\,\,\frac{{dy}}{{dx}},\,\,\,\,\frac{{d\left( {f\left( x \right)} \right)}}{{dx}},\,\,\,\frac{d}{{dx}}\left( {f\left( x \right)} \right)\) (and you may see it differently, such as \(\frac{{df}}{{dx}},\,\,\,\frac{d}{{dx}}y,\,\,\,{{D}_{x}}y\)). It all basically means the same thing.

Note that when you have an algebraic expression that’s a **sum** or **difference** (like \({{x}^{2}}+x+3\)), you can separate the expressions and **take the derivative of each part** (like \({{x}^{2}}\) and *x* and 3) and add (or subtract) them together! This is called the **Sum and Difference Rule**.

Note that there are **examples** for all these rules **here**.

# Constant Rule

This is an easy one; whenever we have a constant (a number by itself without a variable), the derivative is just 0. So if we have *y* = 3 and want the derivative of that function, it’s just 0.

Here is what it looks like in Theorem form:

# Power Rule

The way I remember the **power rule** is take the exponent of a function and move it to the front (to multiply the rest by, including any coefficients), and then take the exponent down a level. In order to use this though, we have to make sure it’s **only x **(or whatever the variable is)

**that’s raised to that exponent**.

So for example, if we have \(y=4{{x}^{3}}\), we move the 3 in front, and bring the *x* cubed down to *x* squared: \(\frac{{dy}}{{dx}}=3\cdot 4{{x}^{{3-1}}}=12{{x}^{2}}\). Interestingly enough, when we take the derivative of the **volume of a sphere** with respect to its radius, \(V=\frac{4}{3}\pi {{r}^{3}}\), we get the **surface area of a sphere**, \(\frac{{dV}}{{dr}}=3\cdot \frac{4}{3}\pi {{r}^{{3-1}}}=4\pi {{r}^{2}}\). Not a coincidence!

Here is what it looks like in Theorem form:

# Product Rule

The **product rule** gets a little more complicated, but after awhile, you’ll be doing it in your sleep. Make it into a little song, and it becomes much easier. And notice that typically you have to use the constant and power rules for the individual expressions when you are using the product rule.

Only use the product rule if there is some sort of variable in both expressions that you’re multiplying. For example, use it when you have something like \({{x}^{2}}\left( {x+3} \right)\), but not something like \(\left( {5{{x}^{2}}} \right)\left( 2 \right)\); turn this into \(10{{x}^{2}}\).

Also, if you can, you can turn exponents on the bottom to **negative exponents**; for example, \(\frac{5}{{{{x}^{2}}}}\,=\,5{{x}^{{-2}}}\) .

So try to **simplify** your function first.

And note that when you have the product of two expressions with variables in it, the derivative is not just the product of their derivatives.

Here’s how I like to remember it: **First times the derivative of the second PLUS second times derivative of the first**. (Yes, that is a PLUS in the middle).

Here is what it looks like in Theorem form:

Note that if you have a **coefficient **in front of two factors, you can either lump the coefficient with one of the factors (like the first one), or take it out and multiply the whole derivative later. For example, for \(y=5x{{\left( {x+1} \right)}^{3}}\), the derivative can be obtained this way:

Note how we took out a greatest common factor (**GCF**) after taking the derivative, in order to simplify the expression.

# Quotient Rule

First of all, remember that you don’t need to use the **quotient rule** if there are just numbers on the bottom – only if there are **variables on the bottom** (in the denominator)!

One thing to remember about the quotient rule is to always **start with the bottom**, and then it will be easier. I remember it this way: **Bottom times the derivative of the top minus top times the derivative of the bottom, all over the bottom squared**. Note that that the top has a minus in it, not a plus.

Note that** if you can separate a quotient into individual terms, it’s best to avoid the quotient rule**. For example, if you have the function \(f\left( x \right)=\frac{{{{x}^{3}}+3}}{x}\), it’s best to divide both terms on the top by *x* to get \(f\left( x \right)={{x}^{2}}+\frac{3}{x}={{x}^{2}}+3{{x}^{{-1}}}\). (You can’t do this with a function like \(f\left( x \right)=\frac{x}{{{{x}^{3}}+3}}\)).

Also note that you can typically turn any function that looks like a quotient into a product, using negative exponents. Some prefer to never use the quotient rule, but to use the product rule instead! But as you practice differentiating, you’ll find the method that works best for you.

Here is what it looks like in Theorem form:

# List of Rules

Here are all the rules, with some examples. Do you see how **with the product and quotient rules**, we may need to **use the constant and power rules**? Also, when we have a (non-variable) **coefficient**, it’s typically easier to take it out first before we do the differentiation.

Note in all these cases, with what we’ve learned so far with these rules, the **coefficient of the x must be 1** (unless we can take out the coefficient from the whole expression).

# More Examples

Here are more examples. Do you see how you need to be really up on your algebra with calculus?

Here is another example of where we have use the **Power Rule twice**, since we’re **multiplying three factors**:

Here are some problems where you have use to the **product and quotient rule**s to find derivatives at certain points using **functions**, or** graphs of functions**:

# Derivatives of Trig Functions

You basically just have to memorize the derivatives of the **basic trig functions**. Here they are:

Here are a few examples; note that the **sum and difference rules** (and all the other rules) apply for trig derivatives, too:

# Higher Order Derivatives

We can actually take the derivative of a function **more than once**; we’ll see this here in the** Equation of the Tangent Line, Tangent Line Approximation, and Rates of Change **Section. We’ll see that the acceleration function is the derivative of the velocity function, which is the derivative of the position function. (In this case, we would take the derivative twice of the position function to get the acceleration function.)

The second derivative (and third derivative, and so on) is what we call a **higher order derivative**, and the notation looks like the following:

(Don’t let all this scare you; you’ll usually just be using one or two types of notation, and it will be pretty obvious).

And with polynomials, if we keep taking derivatives, we’ll eventually end up with 0; for example, for \(y=5{{x}^{4}}-3{{x}^{2}}+2x+3\), we have \({y}’=20{{x}^{3}}-6x+2,\,\,\,\,\,{y}”=60{{x}^{2}}-6;\,\,\,\,\,{y}”’=120x;\,\,\,\,\,\,{{y}^{{\left( 4 \right)}}}=120;\,\,\,\,\,\,{{y}^{{\left( 5 \right)}}}=0\).

Here are more problems; note that in the first case, we have what we call “**indestructible derivatives**” with the sin and cos, since we can keep taking the derivative forever, and the functions never go away (you won’t end up with 0, as in the previous example with a polynomial).

**Understand these problems, and practice, practice, practice!**

Click on Submit (the arrow to the right of the problem) to solve this problem. You can also type in more problems, or click on the 3 dots in the upper right hand corner to drill down for example problems.

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On to **Equation of the Tangent Line, Tangent Line Approximation, and Rates of Change **– you’re ready!