quadratic formula

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hutch27: Master | Next Rank: 500 Posts; Posts: 141; Joined: Fri Sep 07, 2012 10:25 pmFollowed by: 1 member; Location: NY; Thanked: 5 times

by hutch27 » Mon Feb 18, 2013 2:13 pm

whats the deal wit the quadratic formula...will we ever need to use it for factoring on the GMAT?

I heard that it might be good to know the sqr(b^2-4ac) part of the equation to recognize whether something cant be factored, but thats all. Even knowing that seems like itd be a longshot to use on the test.

So should i bother even practicing using it?

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Source: Beat The GMAT — Quantitative Reasoning | Mon Feb 18, 2013 2:13 pm

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ceilidh.erickson: GMAT Instructor; Posts: 2095; Joined: Tue Dec 04, 2012 3:22 pmFollowed by: 247 members; Thanked: 1443 times

by ceilidh.erickson » Tue Feb 19, 2013 9:33 am

The short answer is no, you'll almost definitely never have to use it on the GMAT. Every quadratic that you see on the GMAT should be easy to factor without it.

That said, the quadratic formula is included in the Math Review in the OG, so there's always a chance that they could give you a quadratic that can't be easily factored. It's a very, very, very small chance, though. I have yet to see an official GMAT problem that requires it. If any other experts have, please comment.

If you are good at memorizing, I suppose it wouldn't hurt to memorize this one. If you struggle with memorization, though, I wouldn't spend any time on it if I were you.

Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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dabral: Master | Next Rank: 500 Posts; Posts: 134; Joined: Fri Jan 23, 2009 1:23 pmFollowed by: 15 members; Location: California; Thanked: 59 times

by dabral » Thu Mar 07, 2013 4:11 pm

Here are some cases where the quadratic formula can be useful on the GMAT:

1) Let's say you have the following quadratic: 2x^2+5x-33=0, this particular quadratic came up as part of an official GMAT problem. This can be solved by factoring and that is what I would recommend. However, if you get stuck or cannot do it for any reason, then you can always resort to the quadratic formula(that is some work too), but at least you will have your answer. So the first use is as a backup for solving quadratic equations.

2) On some of the hard GMAT problems you may be tested on being able to apply the concepts based on the quadratic formula. This is only applicable to students targeting 90th percentile plus on the quant. I have seen a few Official GMAT questions in the actual GMAT exam that requires one to know how b^2 - 4ac impacts the nature of the roots and how it relates to whether a parabola intersects the x-axis or not. There are also others that have relied on students knowing how the sum and product of the roots are related to the coefficients of the general quadratic equation ax^2+bx+c=0.

In a nutshell if you are targeting a 90 percentile plus score make sure you fully understand the quadratic formula and how it relates to the roots and intersections of the corresponding parabola.

Cheers,
Dabral

Free Video Explanations: 2021 GMAT OFFICIAL GUIDE.

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