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

1. 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:$\displaystyle \frac{c}{a}$

2. 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

3. Im sorry what?

4. Originally Posted by eh501
Im sorry what?
Sorry I made a typo so it didnt register...look now

5. 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