looking for some quick(I hope) Algebra help

[Fiveohhh]

I've came across this problem a few times and can never remember how to solve it for x, a short little walkthrough would be most appreciated!

x^2+2x=18

I can use my calculator and get -x plus or minus the sqr root of 19 Justcan't remember how to get there. thanks in advance for any help

 

[walla]

just a guess...quadratic formula?

 

[thesurge]

either use the quadratic formula or factor.

 

[TheChort]

quadratic formula FTW

EDIT: and don't forget to write it out as x^2+2x-18=0

 

[Fiveohhh]

Thanks😀

 

[Alphathree33]

x^2+2x=18

x^2+2x - 18 = 0

(x + 1)^2 - 19 = 0

(x + 1)^2 = 19

x + 1 = +- SQRT(19)

x = - 1 +- SQRT(19)

QED

 

[aplefka]

Looks like you've gotten your help. My personal advice is to use the quadratic, even though it's a bitch, because later on in Alg2 there's many times where your brain will laugh at you for trying to factor.

 

[Kyteland]

The "get there" part is the quadratic equation (x = (-b +- sqrt(b^2-4ac))/2a)

The "how to" part is this:
a*x^2+b*x+c=0 // Generic 2nd degree polynomial
x^2+(b/a)*x+c/a=0 // Divide by a
x^2+(b/a)*x=-c/a // Isolate x on the lhs of the equation
x^2+(b/a)*x+b^2/(4*a^2) = b^2/(4*a^2)-c/a // Complete the square
(x+b/(2*a))^2 = (b^2-4*a*c)/(4*a^2) // Complete the square and reduce
x+b/(2*a) = +-sqrt(b^2-4*a*c)/2a // Take the square root
x = (-b +- sqrt(b^2-4ac))/2a // Reduce

 

[aplefka]

Kyteland, would it at all annoy you if I ever PM'd you for pre-cal help? I mean most of the time I can get it answered by my teacher or another teacher, but on the weekends when something comes up it's hard to get help. Feel free to say no, as I'm sure you get enough stupid requests like this already.

 

[IAteYourMother]

Originally posted by: aplefka
Kyteland, would it at all annoy you if I ever PM'd you for pre-cal help? I mean most of the time I can get it answered by my teacher or another teacher, but on the weekends when something comes up it's hard to get help. Feel free to say no, as I'm sure you get enough stupid requests like this already.

precal was rather annoying.

Btw, OP, how old are you?

 

[Kyteland]

Originally posted by: aplefka
Kyteland, would it at all annoy you if I ever PM'd you for pre-cal help? I mean most of the time I can get it answered by my teacher or another teacher, but on the weekends when something comes up it's hard to get help. Feel free to say no, as I'm sure you get enough stupid requests like this already.

That's no problem, but the weekend is when you're least likely to catch me browsing AT. Shoot me any questions by PM, but don't hold it against me if I don't get back to you until Monday. 😉 There's a lot of other "math types" on th board too. If you'll search through some old math question threads you'll see a pattern of who is willing to help out or not. Some of them are even math teachers, too. :Q

Maybe I should download that PM notifier thing so I don't have to log on to get PMs.