avohden wrote:

If x^2 - y^2 = 27, what is the value of (x + y)^2 ?

(1) y = 3

(2) x - y = 3

Target question: What is the value of (x+y)²? Given: x² - y² = 27 Statement 1: y = 3 Take

x² - y² = 27 and replace y with 3 to get: x² - (3)² = 27

Evaluate: x² - 9 = 27

Simplify: x² = 18

So, EITHER x = √18 OR x = -√18

Let's test each case:

Case a: x = √18 and y = 3. So, (x + y)² = (√18 + 3)². So, in this case,

the answer to the target question is (√18 + 3)²Case b: x = -√18 and y = 3. So, (x + y)² = (-√18 + 3)². So, in this case,

the answer to the target question is (-√18 + 3)²NOTE: We need not actually calculate the values of (√18 + 3)² and (-√18 + 3)². We need only see that the two values are DIFFERENT

Since we cannot answer the

target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: x - y = 3 Take

x² - y² = 27 and FACTOR the left side to get: (x + y)(x - y) = 27

Replace (x-y) with 3 to get: (x + y)(3) = 27

So, it must be the case that (x+y) = 9

If (x+y) = 9, then (x+y)² = 81

So,

the answer to the target question is 81Since we can answer the

target question with certainty, statement 2 is SUFFICIENT

Answer: B

Cheers,

Brent

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Brent Hanneson – GMATPrepNow.com

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