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### Nonlinear Factors

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Date: 10/12/98 at 15:28:45
Subject: Factoring (x to the fourth + 4)

I have been told that factoring the sum of two "squared" numbers is
not possible; however, my instructor indicates this one can be done.
I have had no luck. Is he pulling my leg?
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Date: 10/13/98 at 02:00:50
From: Doctor Pete
Subject: Re: Factoring (x to the fourth + 4)

Hi,

First, I'll write:

x^4 + 4

to mean, "x raised to the fourth power, plus 4."  (Similarly, x^3
means "x cubed," and x^2 means "x squared.")

Now, usually, when we think of factoring a polynomial, we are thinking
of finding a form:

(x - a)(x - b)(x - c)...

where a, b, c, ... are constants. But factoring is closely connected
with a similar problem, which is finding roots of a function. In
particular, if you have a polynomial function f(x), and solve the
equation:

f(x) = 0

then what you are finding are the values of a, b, c,... in the factored
form. For example, say:

f(x) = x^2 - 4

Then, observe that f(2) = f(-2) = 0; that is, x = 2, and x = -2 are
roots of f(x). Then f(x) has the factored form:

f(x) = x^2 - 4 = (x - 2)(x + 2)

However, in the case where:

f(x) = x^4 + 4

we see that there is no real value of x for which f(x) = 0, because x^4
is always nonnegative. So one cannot expect a factorization of the
form:

(x - a)(x - b)(x - c)(x - d)

where a, b, c, d are all real numbers. However, it may be possible to
factor it as:

(x^2 + px+ q)(x^2 + rx + s)

that is, as a pair of irreducible quadratics. Why this may be possible
will be clearer in a moment. For now, suppose that:

x^4 + 4 = (x^2 + px+ q)(x^2 + rx + s)

for some unknown constants p, q, r, s, which are all real numbers. By
expanding the righthand side, we see that:

x^4 + 4 = x^4 + (p+r)x^3 + (q+s+pr)x^2 + (ps+qr)x + qs

If we equate the coefficients on both sides of this equation (why?),
then we find that:

p + r = 0
q + s + pr = 0
ps + qr = 0
qs = 4

From the first equation p = -r, and substituting this into the third
equation gives r(-s + q) = 0. This means r = 0, and/or q = s.

Suppose r = 0. Then p = 0, and we have from the second equation that
q + s = 0. But this is impossible, since qs = 4. So we must have
instead that q = s, and hence, from the fourth equation, q = s = 2 or
q = s = -2.

Suppose q = s = -2. Then from the second equation, we have that
-2 - 2 + pr = 0, which implies pr = 4. Since r = -p, we find that
-p^2 = 4, which is impossible.

Now suppose, p = r = 2. From the second equation, we then have
2 + 2 + pr = 0, or pr = -4.  Since p = -r, it follows that r^2 = 4.
Therefore, r = 2 or -2, and p = -2 or 2. Note that this makes sense,
since this produces the factorization:

x^4 + 4 = (x^2 + 2x + 2)(x^2 - 2x + 2)

and clearly p and r are interchangeable, as long as they have opposite
signs. One can check that the factorization is correct by multiplying
out the righthand side.

Now, to gain an understanding of why the polynomial x^4 + 4 factors
into two quadratics but not four linear factors, we attempt to find the
roots of, say, x^2 + 2x + 2, which is one of the quadratics. Clearly,
any root of this polynomial is also a root of x^4 + 4. Using the
quadratic formula, we find that the roots are given by:

x = (-2 + Sqrt[-4])/2 , (-2 - Sqrt[-4])/2
or:
x = -1 + I , -1 - I

where I is the square root of -1. These numbers are complex numbers,
which explains why a linear factorization can't be found. Now, one
could look at the original quartic:

x^4 + 4

and solve for the roots directly: let w = x^2, so we find:

x^4 + 4 = w^2 + 4

Immediately we see that w = {2I, -2I}, and hence x = {Sqrt[2I],
-Sqrt[2I], Sqrt[-2I], -Sqrt[-2I]}. Without going into too much detail,
these values are simply:

x = {-1 + I, -1 - I, 1 + I, 1 - I}

But these complex numbers come in complex conjugate pairs, and when
multiplying and adding complex conjugates, they become real numbers.
So, when we multiply the two linear factors:

(x - (-1 + I))(x - (-1 - I))

for example, we obtain x^2 + 2x + 2, an "irreducible" quadratic.

I hope that made some sense.

- Doctor Pete, The Math Forum
http://mathforum.org/dr.math/
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Associated Topics:
High School Basic Algebra
High School Polynomials
Middle School Algebra
Middle School Factoring Expressions

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