# GMAT Math: Algebra Equations with Radicals

This is a potentially tricky topic.  First, consider these practice questions. 1. Which value(s) of x satisfies the equation above?

I. –1

II. 4

III. 9

• (A) I

• (B) III

• (C) I & II

• (D) I & III

(E) I, II, & III

2. What is the value of y? A full discussion of these problems will come at the end of this article.

## A subtle distinction

Consider the following, very simple algebra equation Of course, this equation has two solutions, +5 and -5.  Of course, we find that by taking the square-root of both sides, remembering this process involves a ± sign.  Now, by contrast, consider this expression: Many folks might think this is, in all respects, the same as the solution in the first process — i.e. “taking the square root.”  We need to draw a subtle distinction here, concerning the nature of this sign: The benighted unfortunately will refer to this as a “square root” sign, but that is a misleading partial name.  The name of this symbol is the “principal square root” sign, where the word “principal”, in the sense of “main” or “primary”, here means the positive root only.  Accordingly, the output of this sign is always positive.

The number 25 has two square roots, one positive and one negative, but it has only one principal square root.  A number can have, at most, only one principal square root.  The principal square root of 25 is +5 only.

When a squared algebraic expression appears in a problem, and we ourselves, in the process of problem-solving, find a square root, we need to include all roots, positive and negative (a common mistake is to forget the negative roots).  BUT, when the symbol above, the principle square root symbol, is printed on the page as part and parcel of the given problem, this means its output will always be positive. ## Solving an equation with a radical

By “equation with a radical”, I will be referring to any algebraic equation in which some of the algebra is under a radical sign, a.k.a a principal square root sign.  The general strategy for such an equation is (a) isolate the radical (i.e. get it alone by itself on one side of the equation); (b) square both sides, thus eliminating the radical; and (c) solve what remains using algebra (often, this will involve factoring a quadratic.)  All well and good, but there’s a catch.

You see, when you square both sides of equation, sometimes that creates solutions that weren’t part of the original equation.  Consider the hyper-simple equation x = 5.  This “equation” has only one solution, positive five.  BUT, if we square both sides, then we get the equation we solved in the previous section, with solutions ±5.  The extra root, x = -5, was not a solution of the original equation, but it became a solution once we squared.  This is an example of an extraneous root —- a number that is not a root of the original equation, but which “becomes” a root when we square both sides.

We have to square both sides to solve an equation with radicals, but doing so introduces the possibility of an extraneous root.  Thus, an essential part of solving any equation with radicals is to check the answers you find, in order to ascertain whether any are extraneous roots.  We verify the roots by plugging them into the original equation —- if the number does not solve the original equation, as given in the problem, then it is not a bonafide solution.

BTW, just as a general point of strategy, regardless of whether radicals are involved, I recommend checking any algebra values you find by plugging them back into the original equation given in the problem, when possible.  It’s just a good habit to check your work.

Extraneous roots play a role in both of the sample problems above.  Having read this post, you may want to go back and give them another attempt before reading the solutions below.  Questions?  Let us know in the comment section at the bottom.

## Solutions to the sample questions (x – 9)(x + 1) = 0

preliminary solutions: x = {+9, –1}

At this point, an unsuspecting student might be tempted to answer (C), the trap answer.  BUT, the problem is: one or both of these answers could be extraneous.  We need to check each by plugging back into the original expression.

Check
x = –1 This answer does not check — the left & right sides have different values.  Thus, x = –1 is an extraneous root, not a solution to the problem.

(NB: it’s often the case that an extraneous root will make the two sides equal to values equal in absolute value and opposite in sign.)

Check
x = 9 The value x = 9 checks — it makes the two sides equal, and thus satisfies the original equation.  This is the only solution, so only option III contains a root.

2) Using a tried and true DS strategy, start with the easier statement, Statement #2. (y – 4)(y + 2) = 0

y = +4 or y = –2

Since there are two values of y, this statement, alone and by itself, is not sufficient.  Lo and behold!  We have arrived at the same equation we found in Statement #2, with solutions y = +4 or y = –2.  The naïve conclusion would be — this statement says exactly the same thing as the other.  That’s incorrect, though, because we don’t know whether both of these values are valid solutions, or whether one or more is an extraneous root.  We need to test this in the original equation.

Test y = +4 This value checks — y = +4 is a valid solution to the equation

Test y = –2 The two sides are not equal, so this does not check!  This value, y = –2, is an extraneous root.

Thus, the equation given in Statement #1 has only one solution, y = 4, so this equation provides a definitive answer to the prompt question.  This statement, alone and by itself, is sufficient.

### 19 Responses to GMAT Math: Algebra Equations with Radicals

1. Sam January 15, 2019 at 9:51 am #

Wouldn’t it have been easier for Q1 to just plug in the numbers from the beginning. I mean you ended up doing that despite going through the whole process of getting rid of the messy radicals

• Magoosh Test Prep Expert January 19, 2019 at 12:30 pm #

Hi Sam!

This is a great question. 🙂 Plugging in the numbers for Q1 right away would be a perfectly fine way to solve the problem. In fact, you’re right, it is the most efficient way to solve the problem! Keep up the great work. 😀

2. Poonam Vijay Gosar June 19, 2018 at 6:30 pm #

The statement 1 has 1 solution i.e. y=4 and statement 2 has two solutions y=4 and y=-2.
So, why is answer not D?
Because there are chances if Statement 1 and 2 being equal, or statement 1 being more than statement 2. Please clarify if I am mistaken.

• Magoosh Test Prep Expert June 27, 2018 at 12:41 pm #

While it’s true that Statements 1 and 2 could be not exactly equivalent in some senses, both Statement 1 and Statement 2 have to be true together. In other words, Statement 2 cannot yield any valid answers that would make Statement 1 untrue. And in Statement 1, the left side of the equation and the right side of the equation, in the equation’s original form, MUST be equal to each other. Plug in +4 for y in the original, un-simplified statement 1 equation, and both sides come out as equal. Plug in -2, and they do NOT come out as equal. So -2 is the only possible value. Using Statement 2 to figure that out is the easiest path to solving this DS problem. However, you can solve Statement 1 on its own if need be, since it’s an algebraic equation with only one variable. So the answer is A– Statement 1 is truly sufficient on its own, while Statement 2 merely provides you with a plug-in shortcut to solve for y using statement 1.

3. Krishna Sarkar September 20, 2017 at 7:43 pm #

Wow! This is one of the best blog i stumbled upon after i made a mistake in the below DS GMAT magoosh question. In hindsight, glad i made that mistake and learnt a great concept!

4. Alejandro September 20, 2017 at 11:34 am #

Hey Mike, I am very thankful about the explanation, I used to struggle with this kind of questions and I did not know why my answers where incorrect. Just a little sugestion, in the first sentence of the solution to the first question, you mentioned that the trap is to pick answer choice (C), however, this does not contain the roots of the equation (-1 and 9) this option contains (-1 and 4).

5. Billy September 17, 2016 at 2:15 pm #

Mike, for statement 2, we get two values, but you never tested them out like you did in statement 1. So for that reason shouldn’t the answer be D? The value of +4 equal 0=0 in statement 1 as well as statement 2 which satisfies the equation of one solution?

• Magoosh Test Prep Expert September 30, 2016 at 9:15 pm #

Hi Billy,

The reason we did not test out Statement 2 is that both values of “x” will work out in Statement 2’s original equation. However, you can definitely test them out yourself. You’ll find that we still don’t have a single answer for “x” as both values work.

However, in Statement 1, if we plug in the two values of “x” in Statement 1’s original equation (with the square root), we will find out that only one value works. This proves that we have only one value of “x”!

6. Prad September 1, 2015 at 8:53 am #

Probably the best post of all the Quant blogs I have read here. I have to admit that I was unaware of this concept. Thanks Mike!

7. Ivana June 6, 2014 at 3:46 am #

Would it be wrong in the first practice question just to plug in directly the values I, II, and III without first find the solutions of the equation?

So it would be the following:

I. -2-2=sqrt16 -4=4
II. 8-2=sqrt61 6=sqrt61
III. 18-2=sqrt256 16=16

• Mike June 8, 2014 at 1:03 pm #

Dear Ivana,
I’m happy to respond. 🙂 That would be a 100% perfectly fine way to solve the problem. In a way, we could say the problem is designed to reward those who simply see to plug in, and punishes some of those who believe they must do algebra whenever they see x. You found the most efficient solution.
Mike 🙂

• Ivana June 9, 2014 at 1:53 am #

Thank you Mike! 🙂

• Mike June 9, 2014 at 12:33 pm #

Ivana,
You are quite welcome, my friend. Best of luck to you!
Mike 🙂

8. Taleya September 6, 2013 at 8:31 am #

Thank you. It was very helpful. I was losing my mind over this issue while solving problems.

• Mike September 6, 2013 at 1:19 pm #

Dear Taleya,
You are quite welcome. I am glad you found it helpful. Best of luck to you.
Mike 🙂

9. Prakash May 13, 2013 at 2:42 pm #

Fantastic Mike! I fell for the trap on both occasions.

• Mike May 13, 2013 at 3:46 pm #

I’m glad you found it helpful! Good luck in the future! Mike 🙂

10. Doreen May 8, 2013 at 1:50 pm #

Awesome.

• Mike May 8, 2013 at 3:58 pm #