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# Three Algebra Formulas Essential for the GMAT

First, a few practice problems.

1) The numbers a, b, and c are all positive.  If , then what is the value of ?

Statement #1: a – b = 3

Statement #2: = 7

2) Given that (P + 2Q) is a positive number, what is the value of (P + 2Q)?

Statement #1: Q = 2

Statement #2:  3) In the diagram above, O is the center of the circle, DC = a and DO = b.  What is the area of the circle?

Statement #1: Statement #2: a + b = 22 4) ABCD is a square with a side y, and JKLM is a side x.  If Rectangle S (not shown) with length (x + y) has the same area as the shaded region above, what is the width of Rectangle S?

(A) x

(B) y

(C) y + x

(D) y – x

(E) ## Three important algebra patterns

Doing math involve both following procedures and recognizing patterns.  Three important patterns for algebra on the GMAT are as follows:

Pattern #1: The Difference of Two Squares Pattern #2: The Squares of a Sum Pattern #3: The Squares of a Difference For GMAT Quant success, you need to know these patterns cold.  You need to know them as well as you know your own phone number or address.  The GMAT will throw question after question at you in which you simply will be expected to recognize these patterns.  In such a question, if you recognize the relevant formula, it will enormously simplify the problem.  If you don’t recognize the relevant formula, you are likely to be stymied by such a question.

## Memory, not memorizing

You might think I would say: memorize them.  Instead, I will ask you to remember them.  What’s the difference?  Memorization implies a rote process, simply trying to stuff an isolated and disconnected factoid into your head.  By contrast, you strengthen you capacity to remember a math formula when you understand all the logic that underlies it.

Here, the logic behind these formulas is the logic of FOILing and factoring.  You should review those patterns until you can follow each both ways — until you can FOIL the product out, or factor it back into components.  If you can do that, you really understand these, and are much more likely to remember them in an integrated way.

## Summary

If these patterns are relatively new to you, you may want to revisit the problems at the top with the list handy: see if you can reason your way through them, before reading the explanations below.  Here’s another practice problem from inside Magoosh:

Do you have questions?  Is there anything you would like to say?  Let us know in the comment section at the bottom!

## Practice Problem Explanations

1) Let X = , the value we are seeking. Notice if we subtract the first equation in the prompt from this equation, we get = X – 117.  In other words, if we could find the value of , then we could find the value of X.

Statement #1: a – b = 3

From this statement alone, we cannot calculate , so we can’t find the value of X.  Statement #1, alone and by itself, is insufficient.

Statement #2: = 7

From this statement alone, we cannot calculate , so we can’t find the value of X.  Statement #2, alone and by itself, is insufficient.

Statements #1 & #2 combined: Now, if we know both statements are true, then we could multiple these two equations, which cancel the denominator, and result in the simple equation a + b = 21.  Now, we have the numerical value of both (a – b) and (a + b), so from the difference of two squares formula, we can figure out , and if we know the numerical value of that, we can calculate X and answer the prompt.   Combined, the statements are sufficient.

Answer = C

2) The prompt tells us that (P + 2Q) is a positive number, and we want to know the value of P.  Remember number properties! We don’t know that (P + 2Q) is a positive integer, just a positive number of some kind.

Statement #1: Q = 2

Obvious, by itself, this tells us zilch about P.  Alone and by itself, this statement is completely insufficient.

Statement #2: Now, this may be a pattern-recognition stretch for some folks, but this is simply the “Square of a Sum” pattern.  It may be clearer if we re-write it like this: This is now the “Square of a Sum” pattern, with P in the role of A and 2Q in the role of B.  Of course, this should equal the square of the sum: All we have to do is take a square root.  Normally, we would have to consider both the positive and the negative square root, but since the prompt guarantees that (P + 2Q) is a positive number, we need only consider the positive root: This statement allows us to determine the unique value of (P + 2Q), so this statement, alone and by itself, is sufficient.

Answer = B

3) To find the area of the circle, we need to use Archimedes’ formula, . For that we need the radius, OC.  We are not given this directly, but notice that r = OC = DC – OD = a – b.  If we knew that, we could find the area of the circle.

Statement #1: A major pattern-matching hit!  This, as written, is the “Square of a Difference” pattern. In fact, this statement already gives us , so we just have to multiply by pi and we have the area.  This statement, alone and by itself, is sufficient.

Statement #2: a + b = 22

We need a – b, and this statement gives us a value of  a + b.  If we had more information, perhaps we could use this in combination with other information to find what we want, but since this is all we have, it’s simply not enough to find a – b.  This statement, alone and by itself, is insufficient.

Answer = A

4) A tricky one.  First of all, notice that the shaded area, quite literally and visually, is the difference of two square — Area = .  We know from the Difference of Two Squares pattern, this factors into:

Area = = (y + x)(y – x)

Well, if a rectangle had this same area, and it had a length of (y + x), it would have to have to have a width of (y – x) — that would make the area the same.  The width has to be (y – x).  Answer = D

#### Special Note:

To find out where algebra sits in the “big picture” of GMAT Quant, and what other Quant concepts you should study, check out our post entitled:

What Kind of Math is on the GMAT? Breakdown of Quant Concepts by Frequency

### 10 Responses to Three Algebra Formulas Essential for the GMAT

1. Hannah November 8, 2018 at 10:43 pm #

The answer to Question 3 doesn’t make sense to me. Isn’t OC equal to OD since it’s a circle and O is in the center?

• Magoosh Test Prep Expert November 13, 2018 at 5:57 pm #

Hi Hannah!

This is a great question. 🙂 Remember – Data Sufficiency diagrams aren’t drawn to scale. Because the length is not explicitly stated, we can’t assume anything about the length of OD even though we have a circle and O is in the center. Therefore, we can’t say that OC is equal to OD. 😀

2. Chayce December 7, 2017 at 11:17 am #

I’m probably missing something here, but before I looked up the answer to problem 1, I worked the problem a different way…probably incorrectly, but I’m not sure why yet.

If we change the first binomial to equal c^2 = 117 – b^2, and then we use that new value to plug into the second given binomial (a^2 + c^2), we get a^2 + (117 – b^2) = 0. So continuing, we move over the “117” to the other side:

a^2 – b^2 = -117

which, factored, looks like:

(a + b)(a – b) = -117

But even before factoring, it looks like the answer to the question is “-117”, without using either of the prompts; obviously, I must be breaking some rule here…maybe setting the equation equal to “0”.

Anyhow, assuming that I must move on the prompts, the first one provide helpful info:

a – b = 3

We can plug this into our factored equation:

(a + b)3 = -117 Divide both sides by 3

a + b = -39 Use prompt one equation to solve for b: (a = 3 + b)

(3+b) + b = -39 Simplify

b = -18 Use to Solve for a

a = -15

Obviously, these don’t satisfy our given given equations, but could you help me figure out where I’ve gone wrong? I assume it has to do with 1) using the stated info : a, b, c are not negative numbers, and 2) trying to combine the given binomials in the way that I did, setting them equal 0.

Any help would be most appreciated; and thanks for all y’all do!

• Magoosh Test Prep Expert December 8, 2017 at 11:56 am #

Hi Chayce,

You hit the nail on the head here–the issue with your response is the fact that you set the equation equal to “0”. We can’t make assumptions like this on the GMAT. We don’t know what this expression is equal to ‘0’ (which is why we set a^2+c^2 equal to X, an unknown variable, in our explanation for this question). It looks like your other problems-solving instincts are OK in methodology (at least, I don’t see anything else strictly ‘illegal’ in your calculations), though as you pointed out they don’t provide you with an answer to the question. It’s difficult to be able to ‘see’ the path forward in questions like this, which is why it’s useful to really study the answers that we give for difficult DS questions–they can help you to understand how to manipulate these questions in a useful (and legal) way 🙂

3. Chris December 6, 2015 at 8:44 am #

Hello,

In the first practice problem we have b^2 + c^2=17.
If we combine the two statements we end up with two equations: a-b=3 and a+b=21. Instead of finding the difference of two squares formula we can use the strategy of elimination(or linear combination) for the aforementioned equations. Thus, a=12 and b=9.
If we substitute b in the initial equation b^2 + c^2=17 => c^2= -64. Nothing on the number line can be squared to yield a negative number. Am i wrong?

• Phuong October 19, 2016 at 6:32 am #

I’ve got the same result. c is impossible here since it is stated that a, b, and c are positive integers. So since we are not able to find out c, E should be the correct answer.

Still, the method makes it seems like we have the right answer.

• Magoosh Test Prep Expert October 20, 2016 at 8:02 am #

Chris and Phoung, it looks like you’re both right that c is an impossible value, which would make E the correct answer after all. However, a “hidden trick question” where the final answer is incalculable due to a negative square is not the sort of thing that you’d see on a real GMAT data sufficiency question. So Mike’s proposed strategy for this question is basically right, in the sense that it would work on the GMAT itself. But the format of the question and the answer to the question are indeed wrong. Thanks for bringing this to our attention, Chris and Phuong. I’ll mention this error to our Content Improvement Team, so they can update the post with a better, more correct initial practice problem.

4. Brittany August 15, 2013 at 4:34 pm #

Hello,

Just wondering if there are supposed to be multiple choice answers for all the practice problems? In the solutions section, with the Answer = A, etc, seems to indicate this, but as far as I can see, the only question that has multiple choice answers is #4. Am I missing something here?

Thanks.

• Mike August 16, 2013 at 9:51 am #

Brittany,
Yes, you are missing something big, in terms of GMAT math. Questions #1-3 are what are called Data Sufficiency questions, which comprise approximately half of the questions on the GMAT Quant section. I’ll recommend these two links to start:
https://magoosh.com/gmat/2012/introduction-to-gmat-data-sufficiency/
https://magoosh.com/gmat/2013/gmat-data-sufficiency-tips/
I don’t know how far along you are in your GMAT prep, but in addition to learning all the math, it is crucial for success on the GMAT to master the Data Sufficiency question format.
Does this make sense?
Mike 🙂

• Daniel September 16, 2015 at 12:43 pm #

Hello,

I think the issue is that those people who are practicing the GRE get linked to here and then the answers aren’t provided in the same format which confuses us. For the GRE it’s usually multiple choice or fill in the blank after all.

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