# Finding the sum+product of a quadratic equation's roots?

• Apr 21st 2008, 04:28 PM
eh501
Finding the sum+product of a quadratic equation's roots?
I wasn't really paying attention in class, and im having trouble. Heres the question:

For each quadratic equation, find: a) the sum of its roots and b) the product of its roots

$\displaystyle x^2-2x-15=0$

Ik in order to find the sum of the roots you need to do:$\displaystyle \frac{-b}{a}$ and for the product:$\displaystyle \frac{c}{a}$
• Apr 21st 2008, 04:32 PM
Mathstud28
Quote:

Originally Posted by eh501
I wasn't really paying attention in class, and im having trouble. Heres the question:

For each quadratic equation, find: a) the sum of its roots and b) the product of its roots

$\displaystyle x^2-2x-15=0$

Ik in order to find the sum of the roots you need to do:$\displaystyle \frac{-b}{a}$ and for the product:$\displaystyle \frac{c}{a}$

Factoring we get $\displaystyle (x-5)(x+3)=0\Rightarrow{x=5,x=-3}$ therefore the sum of its roots is 2 and its product is -15
• Apr 21st 2008, 04:42 PM
eh501
Im sorry what?
• Apr 21st 2008, 05:14 PM
Mathstud28
Quote:

Originally Posted by eh501
Im sorry what?

Sorry I made a typo so it didnt register...look now
• Apr 22nd 2008, 12:34 AM
Moo
Well...

Here, a=1, b=-2, c=-15

The sum of its roots would be $\displaystyle \frac{2}{1}=2$ and the product would be $\displaystyle \frac{-15}{1}=-15$

This is the only thing you're asked (unless you didn't copied everything)

-------

If $\displaystyle a_1$ and $\displaystyle a_2$ are the roots, hence :

$\displaystyle a_1+a_2=2$

$\displaystyle a_1 a_2=-15$

It's better than factorizing from who knows where :)