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555-605 Level|   Algebra|               
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dobinnoli
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Bunuel
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Statement 1:
(x,y)=(5,5) --> x²=xy
(x,y)=(-5,5) --> x²<>xy
not sufficient

Statement 2:
x=y --> xx=xy or x²=xy
sufficient

B
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I agree with bogos.

Stmt1:
x^2 - y^2 = (x+5)(x-5)
x^2 - y^2 = x^2 - 25
0 = y^2 - 25
y = +5, -5

plugin a possible value of y into the equation x^2 - y^2 = x^2 - 25
you can see when that y can be -5 and x can 5. But plugging in the same value into x^2 = xy would yield 25 = -25.
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X=5 =>> y=+/-5
But if you substitute back in eqn only y=5 is valid

Please post OA
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dobinnoli
Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)
(2) x = y

Is x(x-y)=0

So is x=0 or x=y?

Statement 1

Obviously x is not equal to y otherwise LHS and RHS should be zero

Is x = 0

Is -y^2 = -25, could be but we don't know

Statement 2

x=y, yes. This is correct and sufficient

B
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Omg, can someone please post detailed explanation for this one? I don't understand why are we using Y=5 to evaluate the (x+y)(x-y)=(x+5)(y-5) why not use 10 or 40 or something else? why 5? Does it have something to do with question stem?

Im totally confused here
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Omg, can someone please post detailed explanation for this one? I don't understand why are we using Y=5 to evaluate the (x+y)(x-y)=(x+5)(y-5) why not use 10 or 40 or something else? why 5? Does it have something to do with question stem?

Im totally confused here


I guess we are using 5 in order to make (x+y)(x-y) = 0, which is the easiest way to figure out the possible values of x
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dobinnoli
Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)
(2) x = y


PS21275

\(x^2 = xy\)?
\(x^2 - xy = 0\)?
\(x(x-y) = 0\)?
\(x = 0\) or \(x = y\)?

(1) \(x^2 – y^2 = (x + 5)(y - 5)\)
\((x+y)(x-y) = (x+5)(y-5)\)

if \(x = y = 5\), then \(x^2 = xy\)
if \(x = -5\), then \(x^2\) is not equal to xy

INSUFFICIENT.

(2) Directly gives us our answer; SUFFICIENT.

Answer is B.
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x^2 - y^2 = (x+5)(x-5)
x^2 - y^2 = x^2 - 25
0 = y^2 - 25
y = +5, -5

plugin a possible value of y into the equation x^2 - y^2 = x^2 - 25
you can see when that y can be -5 and x can 5. But plugging in the same value into x^2 = xy would yield 25 = -25.

s2

x=y sufficient
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dobinnoli
Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)
(2) x = y
Statement One Only:

x^2 – y^2 = (x + 5)(y - 5)

If y = 5, we have:

x^2 - 25 = (x + 5)(0)

x^2 - 25 = 0

(x + 5)(x - 5) = 0

x = 5 or x = -5

If x = 5, then x^2 and xy will be equal since both are 25. However, if x = -5, then x^2 and xy will not be equal since the former is 25 and the latter is -25. Statement one alone is not sufficient.

Statement Two Only:

x = y

Since x = y, if we multiply both sides by x, we have:

x * x = y * x

x^2 = xy

Statement two alone is sufficient.

Answer: B­
How have we arrived at y = 5?
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dobinnoli
Is x^2 equal to xy?

(1) x^2 – y^2 = (x + 5)(y - 5)
(2) x = y
Statement One Only:

x^2 – y^2 = (x + 5)(y - 5)

If y = 5, we have:

x^2 - 25 = (x + 5)(0)

x^2 - 25 = 0

(x + 5)(x - 5) = 0

x = 5 or x = -5

If x = 5, then x^2 and xy will be equal since both are 25. However, if x = -5, then x^2 and xy will not be equal since the former is 25 and the latter is -25. Statement one alone is not sufficient.

Statement Two Only:

x = y

Since x = y, if we multiply both sides by x, we have:

x * x = y * x

x^2 = xy

Statement two alone is sufficient.

Answer: B­
How have we arrived at y = 5?

It’s not necessary for y to be 5 to satisfy x^2 – y^2 = (x + 5)(y - 5); it could take infinitely many other values. However, y could be 5. If we assume y = 5, we find that x can be either 5 or -5, resulting in two different values for xy. Therefore, identifying a possible value for y (such as y = 5) shows that this statement alone is insufficient.

Hope it's clear.

P.S. Pure algebraic questions are no longer a part of the DS syllabus of the GMAT.

DS questions in GMAT Focus encompass various types of word problems, such as:

  • Word Problems
  • Work Problems
  • Distance Problems
  • Mixture Problems
  • Percent and Interest Problems
  • Overlapping Sets Problems
  • Statistics Problems
  • Combination and Probability Problems

While these questions may involve or necessitate knowledge of algebra, arithmetic, inequalities, etc., they will always be presented in the form of word problems. You won’t encounter pure "algebra" questions like, "Is x > y?" or "A positive integer n has two prime factors..."

Check GMAT Syllabus for Focus Edition

You can also visit the Data Sufficiency forum and filter questions by OG 2024-2025, GMAT Prep (Focus), and Data Insights Review 2024-2025 sources to see the types of questions currently tested on the GMAT.

So, you can ignore this question.

Hope it helps.­
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