Polynomials: Sums and Products of Roots

I want to share with you something interesting about the roots of polynomials ...

What happens when you add the roots?
What happens when you multiply the roots?

Roots of a Polynomial

A "root" (or "zero") is where the polynomial is equal to zero:

Factors

You can take a polynomial, such as:

f(x) = ax⁴ + bx³ + ...

And then factor it like this:

f(x) = a(x-p)(x-q)(x-r)...

Then p, q, r, etc are the roots (where the polynomial equals zero)

Quadratic

Let's try this with a Quadratic:

ax² + bx + c

When the roots are p and q, the same quadratic becomes:

a(x-p)(x-q)

Is there a relationship between p,q and a,b,c?

Let's expand it, by multiplying (x-p) by (x-q):

a(x-p)(x-q)
= a( x² - px - qx + pq )
= ax² - a(p+q)x + apq

Now let us compare:

Quadratic:	ax²	+ bx	+ c
Expanded Factors:	ax²	- a(p+q)x	+ apq

We can now see that bx = - a(p+q)x, so:

p+q = -b/a

And c = apq, so:

pq = c/a

And we get this simple result:

Adding the roots gives -b/a
Multiplying the roots gives c/a

This can help us answer questions.

Example: What is the equation whose roots are 5 + √2 and 5 - √2

The sum of the roots is (5 + √2) + (5 - √2) = 10
The product of the roots is (5 + √2) (5 - √2) = 25 - 2 = 23

And we want an equation like:

ax² + bx + c = 0

When a=1 we can work out that:

Sum of the roots = -b/a = -b
Product of the roots = c/a = c

Which gives us this interesting result

x² - (sum of the roots)x + (product of the roots) = 0

So we end up with:

x² - 10x + 23 = 0

And here is the plot:

Cubic

Now let us look at a Cubic (one degree higher than Quadratic):

ax³ + bx² + cx + d

As with the Quadratic, let us expand the factors:

a(x-p)(x-q)(x-r)
= ax³ - a(p+q+r)x² + a(pq+pr+qr)x - a(pqr)

And we get:

Cubic:	ax³	+ bx²	+ cx	+ d
Expanded Factors:	ax³	- a(p+q+r)x²	+ a(pq+pr+qr)x	- apqr

Hey! We get the same sort of thing:

Adding the roots gives -b/a (exactly the same as the Quadratic)

Multiplying the roots gives -d/a (similar to +c/a for the Quadratic)

(We also get pq+pr+qr = c/a, which can itself be useful.)

Higher Polynomials

The same pattern continues with higher polynomials.

In General:

Adding the roots gives -b/a

Multiplying the roots gives (where "z" is the constant at the end):

z/a (for even degree polynomials like quadratics)
-z/a (for odd degree polynomials like cubics)