# Using Descartes' Rule of Signs

• May 21st 2010, 11:54 AM
Savior_Self
Using Descartes' Rule of Signs
Use Descartes' rule of signs to determine the possible number of:
a) positive real roots of $4x^3 + 2x^2 + 7x + 9 = 0$
b) negative real roots of $4x^3 + 2x^2 + 7x + 9 = 0$

My answer to a) (zero positive real roots), was correct.

Zero positive real roots because there are zero sign changes..

and I had thought there would be zero negative roots as well. But it came back wrong.

Can someone explain to me why? Thanks.
• May 21st 2010, 12:29 PM
masters
Quote:

Originally Posted by Savior_Self
Use Descartes' rule of signs to determine the possible number of:
a) positive real roots of $4x^3 + 2x^2 + 7x + 9 = 0$
b) negative real roots of $4x^3 + 2x^2 + 7x + 9 = 0$

My answer to a) (zero positive real roots), was correct.

Zero positive real roots because there are zero sign changes..

and I had thought there would be zero negative roots as well. But it came back wrong.

Can someone explain to me why? Thanks.

Hi Savior_Self,

$f(-x)=4(-x)^3+2(-x)^2+7(-x)+9$

$f(-x)=-4x^3+2x^2-7x+9$

Now, how many sign changes do you see?
• May 21st 2010, 12:31 PM
spruancejr
To find the maximum number of positive real roots count the number of sign changes. To find the number of negative real roots replace all x values with a (-x) and perform the same operation of counting sign changes.

For example,

$x^2+x+2$ becomes...
$(-x)^2+(-x)+2$
• May 21st 2010, 12:42 PM
ebaines
Using Descarte's method, to determine the number of negative roots you replace the x in the polynomial with -x, and see how many sign changes there are in the resulting equation:

$
4(-x)^3 + 2(-x)^2 + 7(-x) + 9 =
$

$
-4x^3 + 2x^2 - 7x + 9
$

Note that there are 3 changes of sign as you read left to right. Thus there may be 3 negative roots, or possibly just 1 (since it's possible that 2 of the roots may actually be complex).

If you plot this function you'll see that there is one real negtive root. The two other roots are therefore complex.
• May 21st 2010, 12:56 PM
Savior_Self
Awesome. Thanks guys. I didn't pick up on the x to (-x) step for some reason. I won't forget it now.
• May 21st 2010, 01:47 PM
ebaines
There's another "trick" that you could have used. Remember that a polynomial of order N must have N roots. So since you already knew that there were zero positive roots, you know that there must be either (a) 3 negative roots, or (b) 1 negative root and 2 complex roots.