Recognizing disguised quadratics in problems (generally)

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DCS80: Senior | Next Rank: 100 Posts; Posts: 33; Joined: Mon Dec 03, 2012 6:08 pmFollowed by: 2 members; Thanked: 2 times

by DCS80 » Thu Jan 17, 2013 1:48 pm

Hi Guys,

reviewing the perversities of quadratics, I was curious as to if there is a list/sample of the most commonly rephrased equations

for example,

(36/b) = b-5
or
3w^2 = 6w

maybe a small collection of equations that can be rewritten as clear quadratic equations?

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Source: Beat The GMAT — Problem Solving | Thu Jan 17, 2013 1:48 pm

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Tommy Wallach: GMAT Instructor; Posts: 451; Joined: Thu Jan 21, 2010 11:58 amFollowed by: 120 members; Location: New York City; Thanked: 188 times; GMAT Score:770

by Tommy Wallach » Thu Jan 17, 2013 11:41 pm

Hey DCS80,

I wouldn't attempt to find a long list of quadratics, because you'll just be wasting your time. Instead, try to notice the most typical signals. There are the three major identities: 1) (x+y) * (x+y) 2) (x-y) * (x-y) and the most important, 3) (x+y) * (x-y).

The third identity (sometimes called "Difference of squares") comes up constantly on tough GMAT questions, so you need to always be on the lookout for it in different forms.

As for the examples you give, those aren't situations you need to memorize. Anytime you have something in the form of ax^2 + bx + c, you're dealing with a quadratic. Your first example will clearly become that form when you multiply both sides by b (which is the logical first step to take). The second example may be slightly more confusing, because it's missing the c part of the quadratic (technically, c = 0), but it should still set off alarm bells when you have ax^2 + bx.

Does that help? Let me know if you have more questions!

-t

Tommy Wallach, Company Expert
ManhattanGMAT

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DCS80: Senior | Next Rank: 100 Posts; Posts: 33; Joined: Mon Dec 03, 2012 6:08 pmFollowed by: 2 members; Thanked: 2 times

by DCS80 » Fri Jan 18, 2013 10:04 am

awesome, thanks. any particular examples for the difference of squares reference?

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GMAT/MBA Expert

Tommy Wallach: GMAT Instructor; Posts: 451; Joined: Thu Jan 21, 2010 11:58 amFollowed by: 120 members; Location: New York City; Thanked: 188 times; GMAT Score:770

by Tommy Wallach » Fri Jan 18, 2013 2:38 pm

Hey DCS80,

This page has some tips and tricks: https://www.manhattangmat.com/strategy-s ... -roots.cfm As for questions, there are plenty out there, but likely only a few in the official guides. I recommend, after studying a bit, see if you can page through the official guide and pick them out by sight (then you can check in the back to see if you're right).

Good luck!

-t

Tommy Wallach, Company Expert
ManhattanGMAT

If you found this posting mega-helpful, feel free to thank and/or follow me!

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