# havent a clue!

• Dec 21st 2007, 12:26 PM
jenko
havent a clue!
havent a clue!

6x2+x-15=0
• Dec 21st 2007, 12:37 PM
topsquark
Quote:

Originally Posted by jenko
havent a clue!

6x2+x-15=0

Let's try this using the method I posted in your other thread.

Mulitply 6 and -15: -90

Now list all pairs of factors of -90:
1, -90
2, -45
3, -30
6, -15
9, -10
10, -9
15, -6
30, -3
45, -2
90, -1

Now which of these pairs add to 1? The 10 and -9, of course. So 1 = 10 - 9:
$6x^2 + x - 15 = 0$

$6x^2 + (10 - 9)x - 15 = 0$

$6x^2 + 10x - 9x - 15 = 0$

$(6x^2 + 10x) + (-9x - 15) = 0$

$2x(3x + 5) - 3(3x + 5) = 0$

$(2x - 3)(3x + 5) = 0$

So set each factor equal to 0:
$2x - 3 = 0 \implies x = \frac{3}{2}$
or
$3x + 5 = 0 \implies x = -\frac{5}{3}$

-Dan
• Dec 21st 2007, 12:42 PM
colby2152
BTW, after a lot of practice of factorials, you will be able to look at them for a couple seconds and know what the constants will be.

I understand your plight. When I was a kid learning about factoring, my teacher didn't even show me about the additives and multiples that make up the constants right away. Needless to say, I taught it to myself.(Tmi)
• Dec 21st 2007, 12:52 PM
jenko
i know the basics of mathematics e.g multiples etc it jus some of the terminologies that are used which i have never heared of or are unsure of ite been a while since i have done this last time i done then wen i was in high school two years ago and ever since that day i forgot it all as i thought i would never use it on it later life which i realy regret otherwise i wouldnt be here now or better still would be here with even more harder questions for you guys to help me with! :D