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Quadratic Equation using formula

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by Brent@GMATPrepNow » Tue Nov 27, 2012 3:59 pm
soumya_165 wrote:Hi All

Can anyone please help me out by solving the following equation using the formula

X^2+20X-8000

Formula: -b+-((b^2 - 4ac)^1/2)/2a
I'll start by mentioning that X^2+20X-8000 is not an equation.
I think you mean x^2 + 20x - 8000 = 0

I've never seen an official GMAT question that requires us to use of the Quadratic formula to solve an equation (even though the formula appears in the math review section of the Official Guide), but here goes . . .

The quadratic equation says:
If ax^2 + bx + c = 0, then x = [-b + sqrt(b^2 - 4ac)]/2a and x = [-b - sqrt(b^2 - 4ac)]/2a

In the given equation, a = 1, b = 20 and c = -8000

So, the two solutions are:
1) x = [-20 + sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 + 180]/2
= 160/2
= 80

2) x = [-20 - sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 - 180]/2
= (-200)/2
= -100

So, the solutions are x = 80 and x = -100

Cheers,
Brent
Last edited by Brent@GMATPrepNow on Tue Nov 27, 2012 4:03 pm, edited 1 time in total.
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by Brent@GMATPrepNow » Tue Nov 27, 2012 4:01 pm
The faster approach is to factor.

x^2 + 20x - 8000 = 0
(x + 100)(x - 80) = 0

So, the solutions are x = 80 and x = -100

Cheers,
Brent
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by soumya_165 » Wed Nov 28, 2012 7:01 am
Thanks Brent.. I used the Factorization method and got the answer. I just wanted to solve it using the formula but was making a very silly mistake for which I was not getting the answer and was stuck. Thanks for helping me out by solving this..
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by lazarogb » Sun Dec 09, 2012 6:18 am
Brent@GMATPrepNow wrote:
soumya_165 wrote:Hi All

Can anyone please help me out by solving the following equation using the formula

X^2+20X-8000

Formula: -b+-((b^2 - 4ac)^1/2)/2a
I'll start by mentioning that X^2+20X-8000 is not an equation.
I think you mean x^2 + 20x - 8000 = 0

I've never seen an official GMAT question that requires us to use of the Quadratic formula to solve an equation (even though the formula appears in the math review section of the Official Guide), but here goes . . .

The quadratic equation says:
If ax^2 + bx + c = 0, then x = [-b + sqrt(b^2 - 4ac)]/2a and x = [-b - sqrt(b^2 - 4ac)]/2a

In the given equation, a = 1, b = 20 and c = -8000

So, the two solutions are:
1) x = [-20 + sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 + 180]/2
= 160/2
= 80

2) x = [-20 - sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 - 180]/2
= (-200)/2
= -100

So, the solutions are x = 80 and x = -100

Cheers,
Brent
Hello Brent,

you might wanna check out a problem I just ran into with the gmat prep software extension package.

I think the gmat people expect us to know and be able to use the quadratic formula.

I posted it here:

https://www.beatthegmat.com/quadratic-fo ... tml#539832

thanks!
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Posts: 16207
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by Brent@GMATPrepNow » Sun Dec 09, 2012 9:20 am
lazarogb wrote:
Brent@GMATPrepNow wrote:
soumya_165 wrote:Hi All

Can anyone please help me out by solving the following equation using the formula

X^2+20X-8000

Formula: -b+-((b^2 - 4ac)^1/2)/2a
I'll start by mentioning that X^2+20X-8000 is not an equation.
I think you mean x^2 + 20x - 8000 = 0

I've never seen an official GMAT question that requires us to use of the Quadratic formula to solve an equation (even though the formula appears in the math review section of the Official Guide), but here goes . . .

The quadratic equation says:
If ax^2 + bx + c = 0, then x = [-b + sqrt(b^2 - 4ac)]/2a and x = [-b - sqrt(b^2 - 4ac)]/2a

In the given equation, a = 1, b = 20 and c = -8000

So, the two solutions are:
1) x = [-20 + sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 + 180]/2
= 160/2
= 80

2) x = [-20 - sqrt(20^2 - (4)(1)(-8000)]/[2(1)]
= [-20 - 180]/2
= (-200)/2
= -100

So, the solutions are x = 80 and x = -100

Cheers,
Brent
Hello Brent,

you might wanna check out a problem I just ran into with the gmat prep software extension package.

I think the gmat people expect us to know and be able to use the quadratic formula.

I posted it here:

https://www.beatthegmat.com/quadratic-fo ... tml#539832

thanks!
The equation posted on that link is tougher than most equations, but we can still solve it by factoring.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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