GMAT Math: The Uses and Abuses of Formulas

Question

GMAT Math: The Uses and Abuses of Formulas  
  BY MIKE MCGARRY, MAGOOSH

Formulas are useful

Clearly, it’s a good idea to know some  GMAT math formulas  —– many of the best formulas to know are listed on that blog. Formulas can be wonderful shortcuts, very efficient time-savers. Clearly it would be a mistake to walk into the GMAT Math section without knowing a single formula.

But BEWARE!

Formulas are useful to a point, but one of the worst mistakes folks make when they study for GMAT Math is to think that   all   they have to do to master the GMAT Math section is to memorize all the formulas. Similarly, the worst way to approach a GMAT math section is to approach each question asking only,  “What’s the best formula to use for this question?”  That’s roughly the same as thinking that all I need to be the world’s best  lion tamer  is a fancy hat that says “lion tamer”! In fact, the GMAT regularly designs math questions that predictably punish folks who blindly memorize formulas. Most of the harder math problems on the GMAT have this property: if you approach the problem with the perspective of “what’s the best formula to use to solve this question?”, then you very likely will miss what the question is really about and get it wrong. Here are some tips you can use to escape these traps.

Remember, but don’t memorize

What do I mean by this? Yes, you have to remember some formulas for the GMAT. One option would be to memorize each one separately as an isolate mathematical factoid. Some people are exceptionally good memorizers, but for most people, this is not too helpful: in particular, if, in the stress of the real GMAT, you forget what you memorized, then you are out of luck. Memorizing allows you no recourse if you forget, and it gives you no insight into mathematical logic.

Instead, I would recommend: understand the derivation of every formula: that is, understand the logical argument that underlies the formula and from which the formula arises. Think in terms of “why is it true?”: don’t stop at simply “what is true?” For example, both the  permutation  and  combination  formulas can be derived directly from the  Fundamental Counting Principle . In another blog, I talked about the derivation of the formula for  the area of an equilateral triangle . For most of the formulas you need to know for the GMAT, there is a context of logical argument that is also helpful to know. One exception is  Archimedes‘  formula for the area of a circle:
 A=\pi{r^2} 
Yes, the great master Archimedes had a brilliant argument for why that formula is true, but that’s well beyond the kind of logic you need to employ on the GMAT. For this one, you can take a pass and simply memorize the formula, if you haven’t already. For most formulas, though, you should explore the logical of the context in depth.

Mathematical thinking

Problem-solving in mathematics is a subtle and sophisticated subject, and even in the limited context of the GMAT math section, the harder questions sometimes demand insightful approaches. Formulas are very cookie-cutter: if you have a formula to do something, it’s very good at doing that one thing, but that one thing is all it can do. That’s quite different from the flexible analyses characteristic of  mathematical thinking .

Think about it this way. When you approach the GMAT math section, you should have a well-equipped tool box. The individual formulas you know are tools, but each one is a highly specific tool, useful for only one thing. Other more widely applicable tools are skills such as  estimation  or  backsolving  or  picking numbers . It’s important to have in your “tool box” a mix of tools, some more general and some more specific; what’s more important, though, it knowing when and how to use them.

Perspective

In the post  How to do GMAT Math Faster , I talk in depth about left-brain vs. right-brain thinkers. Left-brain dominant thinkers want to know  “what to do" , and they love formulas, because it’s always very clear what to do with a formula. A more right-brain perspective focuses on  “how to look at the problem“ , on the question of the best way to frame a problem, the best problem-solving perspective to adopt. Often, on a challenging GMAT problem, when one adopts the best perspective, what to do becomes obvious. Given the correct perspective, many test-takers could solve the problem, but most folks get that problem wrong because it’s hard to come up with that right perspective on one’s own. Coming up with the best perspective for a problem, the best way to frame a problem, takes time. It’s a right-brain pattern-matching skill: these always take time & experience to master. It’s very important to read solutions carefully: left-brain thinkers will want to jump ahead to the formulas, to “what to do“, and may be frustrated because they knew all those parts already. To get the most from solutions, it’s very important to study the very beginning, any reasons given for making one perceptual choice rather than another. Sometimes, it may be helpful to solve the problem in more than one way, to experience the difference in different solution routes. Sometimes, the perceptual choices are hard to get from the solution, and the only way to find an answer is to post a question to an expert in the forums, asking them:  “why do we get the solution this way but not in that way?”  Through asking questions and investigating, over time one develops more of a sense for problem-solving perspective.

Summary

Yes, know the GMAT formulas, but don’t think they are a magic bullet. You will get considerably more from understanding the logic behind each formula, than you will from blind memorization of only the formula. Finally, it’s most important to develop the problem-solving perspective, which will help you to see which formulas to use when.

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