1) Gives you various single values for x and y. Therefore clearly insufficient.
2) If \(\sqrt{xy}= 6\), then xy = 36 which can be built with 3*12 or 6*6 ... insufficient.

1+2) Here we know, x+y = 15 and xy = 36, hence x, or y are splitted up as 12 and 3. It does actually not matter if x is 3 or 12 or y is 3 or 12. The sum of \(\sqrt{x} + \sqrt{y}\) will be the same.

Answer C.

((x)^(1/2)+(y)^(1/2))^2 = x + y + 2 *(xy)^(1/2)

1. x+y= 15
Not sufficient

2.
(xy)^(1/2) = 6
Not sufficient

Combining 1 and 2, we get
x + y + 2 *(xy)^(1/2)= 15 + 2*6=27
=> ((x)^(1/2)+(y)^(1/2))^2 = 27
=> (x)^(1/2)+(y)^(1/2) =3*((3)^(1/2)

Sufficient
Answer C
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Whenever we are given that for example, root(n) = something,

Can we always pretty much blindly conclude that n = (something)^2?

Or is there something we have to watch out for.

Sorry to post little unrelated post; where should we consider mode in GMAT. As I remember, in one of your post, you mentioned -- sqrt(x^2)=|x|

Target question: What is the value of √x + √y?

Statement 1: x + y = 15
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several values of x and y that satisfy statement 1. Here are two:
Case a: x = 14 and y = 1, in which case √x + √y = √14 + √1 = √14 + 1
Case b: x = 9 and y = 6, in which case √x + √y = √9 + √6 = 3 + √6
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Aside: For more on this idea of plugging in values when a statement doesn't feel sufficient, read my article: http://www.gmatprepnow.com/articles/dat ... lug-values

Statement 2: √(xy) = 6
In other words xy = 36
This statement doesn't FEEL sufficient either, so I'll TEST some values.
There are several values of x and y that satisfy statement 2. Here are two:
Case a: x = 1 and y = 36, in which case √x + √y = √1 + √36 = 1 + 6 = 7
Case b: x = 4 and y = 9, in which case √x + √y = √4 + √9 = 2 + 3 = 5
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Statement 1 tells us that x + y = 15
Statement 2 tells us that √(xy) = 6
Recognize that (√x + √y)² = x + 2√(xy) + y
Rearrange to get: (√x + √y)² = x + y + 2√(xy)
We get: (√x + √y)² = 15 + 2(6)
Evaluate: (√x + √y)² = 27
So, √x + √y = √27
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer: C

RELATED VIDEO

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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.


Since we have 2 variables (x and y) and 0 equations, C is most likely to be the answer and so we should consider 1) & 2) first.

\((\sqrt{x} + \sqrt{y})^2 = x + 2\sqrt{xy} + y = x + y + 2\sqrt{xy} = 15 + 2 \cdot 6 = 15 + 12 = 27\).
Both conditions 1) & 2) are sufficient.

Since this is an integer question (one of the key question areas), we should also consider choices A and B by CMT 4(A).

Condition 1)
Since \(y = 15 - x\), we have \(\sqrt{x} + \sqrt{y} = \sqrt{x} + \sqrt{15 - x}\).
However, the condition 1) is not sufficient since we don't know \(x\).

Condition 1)
Since \(xy = 36\) and \(y = \frac{36}{x}\), we have \(\sqrt{x} + \sqrt{y} = \sqrt{x} + \sqrt{\frac{36}{x}}\).
However, the condition 1) is not sufficient since we don't know \(x\).

Therefore, C is the answer.

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
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The square root of a number (generally even root of a number) is non-negative: 0 or positive. \(\sqrt[even]{nonnegative \ number}\geq 0\). Thus, \(\sqrt{x} + \sqrt{y} = {nonnegative \ number} + {nonnegative \ number}= {nonnegative \ number}\), so it cannot equal to a negative number.
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I thought (1) alone was enough since if
x+y=15
I could do the squared root of each term
√x + √y = ± √15
Am I breaking some math rules?

Thank you in advance

Bunuel chetan2u niks18 amanvermagmat




I think we over complicated the solution.
Can we not simply approach q as : \((a+b)^2\) = \(a^2\)+ \(b^2\) + 2 ab

Substitute \(\sqrt{a}\) for a and \(\sqrt{b}\) for b
only both statements together help in completing equation.

Is this method correct?
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hello there GMATPrepNow

can you please explain how you combine both statements to get answer

Statement 1 tells us that x + y = 15
Statement 2 tells us that √(xy) = 6

and then you say Recognize that (√x + √y)² = x + 2√(xy) + y

I dont recognize this pattern in either of the statements how can I recognize it looking at both statements where did you get this formula ?

I thought I should do square both sides of √(xy) = 6 so I am getting xy = 36 and also have x + y = 15 so I do something like this

x + y = 15 ---> x = 15-y and plug in here xy = 36

thank you in advance for taking time to explain and have a great gmat weekend