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The power of x

Powers of x and higher polynomials

There is a family of relations that includes linear, quadratic and inverse relations all in the same framework. This is the family of functions defined by a power of .

In this step we look at different powers of , and see how they are the building blocks of polynomials.

Linear, quadratic and cubic powers

You already know about linear powers, such as , quadratic powers such as and now also cubic powers such as . Here are their graphs:

Graphs of y=x, y=x^2 and y=x^3 in the range [-12,14]

It is important to understand how higher powers of behave. The general pattern follows the three functions above, but generally as the exponent increases, the function becomes more extreme — and it gets very big more quickly as increases.

Higher powers of

Recall that for any . So the function is the same as the function — that is, it is a constant function that does not change at all. In the following diagram, you can see the functions for and how they grow with .

Plots of x^n for n=0,1,2,3,4,5. All curves go through the origin. Some have all positive values and some have both positive and negative values. Some increase more quickly than others.

Q1 (E): In the above graph, which function is which?

When the degree of the function is an odd natural number, such a polynomial always increases as increases. In this case we say that is an increasing function. However if is even, then as increases, two things can happen: the function decreases while is negative, and then increases for positive .

Index laws

When working with powers it is useful to remember the index laws

and for good measure we also include the rule that lets us work with negative exponents:

We will shortly be talking about powers with fractional exponents. It will then be important to remember that these index laws still hold.

Growth rates of powers

The differences between various powers often becomes more noticeable for very small or very large values of . While there is not so much difference between and , there is quite a big difference between and , and also between and .

We say that the power has a larger growth rate than . This becomes ever more noticeable as the exponent in increases.

Q2 (E): Which has a faster growth rate: the function or ?

Q3 (M): What about when is getting closer to zero? Which of the functions and will decrease faster?

Combining powers of

The simplest polynomials are the power functions for natural numbers , and they form the basic building blocks to make more general polynomials, such as .

These are an important class of objects, because they are exactly the natural domain of algebra: we can add, subtract and multiply polynomials in much the same way as we do arithmetic with numbers, and sometimes we can also divide them. But polynomials are considerably more complicated than numbers, especially as we increase the degree.

Nevertheless, the basic guiding principle — that polynomials give us a domain of arithmetic that extends that of ordinary numbers — is a powerful and useful one.

Polynomials as products of factors

There is another way of getting polynomials rather than combining multiples of powers of . Just as one way of getting bigger numbers is to multiply them together (for example ) , so too for polynomials we can multiply smaller polynomials together to get higher degree polynomials. The simplest way of doing that is to multiply together linear factors, for example

By Descartes’ theorem (Factors of quadratic polynomials and zeroes) each linear factor of corresponds to a zero of the function, so for example in this case .

You should be aware that a polynomial that has been composed as a product of linear factors is arithmetically rather special. In general a polynomial of degree may have less than linear factors. For example has no linear factors.

Shapes of polynomial functions

As we go up in degree, polynomial functions get longer to write down, with more coefficients, and their graphs become more complicated. Nevertheless there is a kind of predictability about the overall shape of polynomials that it is essential for us to understand. Even a higher degree polynomial function such as

has something of a regular graph: as we view it from left to right, it goes up, then down, then up, then down and then finally up.

Graph of the curve described above, with a line intersecting this curve at 5 points

The number of ups and downs is limited by the degree. A fundamental fact about polynomials is that the graph of a degree n polynomial can meet an arbitrary line in at most n points. This is an important theorem. Another variant is that a polynomial of degree can change direction — from up to down, or from down to up, as we view it from left to right — at most times.

But remember that not all polynomial functions exhibit this up, down, up, down aspect explicitly. In particular the powers of that we began our discussion with only go up if is odd, and go down and then up if is even, no matter how high the degree .

Answers

A1. The function is light blue (the horizontal line), is red (the straight line going through the point ), is yellow (one of the 2 ‘even’ functions — with all positive values), is orange (one of the 3 ‘odd’ functions — with negative values for x < 0), is green (again an even function) and is dark blue (again an odd function).

A2. Since is greater than the function grows faster than as gets larger. The coefficients are relatively unimportant, so that also grows faster than .

A3. Since is greater than the function decreases faster than as gets smaller. So also decreases faster than as gets smaller.

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This article is from the free online course:

Maths for Humans: Inverse Relations and Power Laws

UNSW Sydney

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