4.01 Introduction to anti-differentiaion

We can use differentiation to find the derivative, which we have seen has many useful applications. What if we were given a derivative function, can we go backwards to find the original function it came from? Can we "anti-differentiate"?

For example, if we know that the derivative of some function $f\left(x\right)$f(x) is $6x+4$6x+4, can we find what $f\left(x\right)$f(x) is? What the degree of $f\left(x\right)$f(x) is? Do we have enough information?

Let's explore this problem by first finding the derivative of the following five functions:

What do you notice?

Well, since the constant terms $5,1,-17,-3$5,1,−17,−3 and $a$a all have a derivative of zero, we end up getting the same gradient function for all of them: $F'\left(x\right)=6x+4$F′(x)=6x+4.

So, if we are given that the gradient function of $f\left(x\right)$f(x) is $6x+4$6x+4 and we are looking for the original function, how do we know which one it is? Without extra information, such as a point the original function passes through, we cannot tell what the constant term may have been.

In fact, when we go backwards using anti-differentiation, we cannot be sure which function was the original so we find what is called a primitive function. We can give the form of all possible primitive functions by adding an unknown constant.

For example, all primitive functions of $6x+4$6x+4 have the form $F\left(x\right)=3x^2+4x+c$F(x)=3x2+4x+c, where $c$c is a constant term that we don't yet know the value of. When including the constant we often refer to the form as the primitive function, and this is also known as the general anti-derivative or the indefinite integral.

The process of "going backwards" from differentiation to find a primitive function is called anti-differentiation or indefinite integration.

For power functions of the form $f\left(x\right)=ax^n$f(x)=axn, our differentiation rule tells us $f'\left(x\right)=nax^{n-1}$f′(x)=naxn−1. That is, "Multiply the expression by the power of $x$x and then subtract $1$1 from the power." For anti-differentiation we want to "undo" that. Reversing the rule we get "add $1$1 to the power of $x$x, then divide by the expression by the new power"

So for $f\left(x\right)=ax^n$f(x)=axn, the indefinite integral is $F\left(x\right)=\frac{ax^{n+1}}{n+1}+c$F(x)=axn+1n+1​+c. The value $c$c (can also be written as $C$C )is known as the constant of integration.

Notation: The expression $\int f\left(x\right)\ dx$∫f(x) dx, reads as "anti-differentiate the function $f\left(x\right)$f(x) with respect to $x$x." For example, $\int3x^2dx=x^3+c$∫3x2dx=x3+c, where $c$c is a constant, reads as "the general anti-derivative of $3x^2$3x2 with respect to $x$x is $x^3+c$x3+c".

Worked examples

Example 1

Find the primitive function $F\left(x\right)$F(x) when $f\left(x\right)=3x^2+4x$f(x)=3x2+4x.

Think: Term by term, we raise the power by one and divide by the new power.

Do:

Example 2

Given that $\frac{dy}{dx}=\frac{1}{x^3}-\sqrt{x}$dydx​=1x3​−√x, find the form of all anti-derivatives $y$y.

Think: First rewrite all terms as powers of $x$x and then integrate term by term.

Do: $\frac{dy}{dx}=x^{-3}-x^{\frac{1}{2}}$dydx​=x−3−x12​

Example 3

Given that $f'\left(x\right)=6x^2-4x+1$f′(x)=6x2−4x+1 and $f\left(x\right)$f(x) passes through the point $\left(2,7\right)$(2,7), find the function $f\left(x\right)$f(x).

Think: Find the indefinite integral and then use the given point to find the constant for the given function.

Do:

Given $f\left(x\right)$f(x) passes through $\left(2,7\right)$(2,7), we have:

Hence, $f\left(x\right)=2x^3-2x^2+x-3$f(x)=2x3−2x2+x−3.

Practice questions

Question 1

Question 2

Question 3

Question 4