Quadratic equation?

Question

This question is driving me mad and I can't figure it out...
it says use the quadratic equation to solve, x^2 - 3x - 18 = 0
i've been trying for the past half hour and I can't figure it out, any help is appreciated.

Thank you all for the helpful links

Answer 1

x² - 3x - 18 = 0

The middle term is - 3x

Find the sum of the middle term

Multiply the first term 1 times the last term 18 equls 18 and factor

Factors of 18 =

1 x 18
2 x 9
3 x 6. . .<= use these factors

+3 and - 6 satisfy the sum of the middle term

insert + 3x and - 6x into the equation

x² - 3x - 18 = 0

x² + 3x - 6x - 18 = 0

x(x + 3) - 6(x + 3)

(x - 6)(x + 3)

- - - - - - - - -s-

Answer 2

You mean use the quadratic formula? If you have a quadratic equation in the form of ax² + bx + c = 0, then the quadratic forumula says
x = [ -b ± √(b²-4ac) ] / 2a

So using the forumla here, the solution is:
x = [3 ± √(9 - 4*1*(-18)) ] / 2
x = [3 ± 3√(1 - 4(-2)) ] / 2
x = [3 ± 3√(9) ] / 2
x = [3 ± 9] / 2
So x = (3+9)/2 and (3-9)/2, or simplified,
x = -3, 6

By the way, you can derive the quadratic formula by applying completing-the-square on the general quadratic ax² + bx + c = 0.

Answer 3

That is a quadratic equation. You sure it doesn't say formula?The quadratic formula is: x= (-b +or- sqr(b^2-4ac))/2a
Where a, b, and c are the coeficients in a quadratic equation.
Therefore a=1 b=-3 and c=-18
Giving:
x=( - -3 +or- sqr(9- -72))/2
x= (3 +or- sqr 81 ) /2
X= (3 +or- 9)/2

so x = -6/2 or 12/2
x= -3 and/or 6

TO check:
NOTE: YOU MUST DO THIS PART TO DETERMINE IF BOTH ROOTS ARE FEASIBLE!! They are not always!

-3^2-3*3-18=?
9-9-18=0? NO

6^2 - 3*-6 -18=?
36-18-18=0 YES!

So the (only) correct answer/root is 6.

Answer 4

Hopefully someone will give you a better answer, but you can read about the quadratic equation here:

http://en.wikipedia.org/wiki/Quadratic_equation

And from my limited math memory, the coefficients are the numbers next to the variables. So your coefficients are 1, -3, and -18, I think. A= 1, B= -3, and C= -18.

May God bless you.

Answer 5

Yep - you need to know the "quadratic equation"
It's an actual equation you will learn to memorize

it is:
x=-b +/- [b^2 - [square root (4ac)] and all divided by 2a

+/- means "plus or minus"
see it here: http://www.math-help.info/quadratic%20equation%206.jpg

It uses the generally form of ax^2+bx+c
so in ur case....a=1, b=-3 and c=-18

so just plug into the equation to find x

Answer 6

it's been so long since i've looked at a quadratic equation but i know how i'd always wished i could post something online and get an answer...before yahoo answers came along..
try this.. http://www.1728.com/quadratc.htm

Answer 7

6, -3

Answer 8

-b+-((b^2-4ac)/2)^1/2 coeficient of first x is 1,second is -3, and -18, the eqution is aX^2+ bX +c, I hope you can plug and solve.!!

Answer 9

(x-6)(x+3)=0
x=6 or -3

Answer 10

x=-3 x=6
Please give me best answer thanks!