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Bunuel
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x^2-8xy+16y^2 = 49

(x-4y)^2 = 49

|x-4y| = 7

As x and -4y are both negatives, |x-4y|= 4y-x

So, 4y-x = 7

x= 4y-7

Ans E

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Put x = -x in the given equation:

\((-x)^2 - 8 (-x)(y) + 16y^2 = 49\)

=> \(x^2 + 8xy + 16y^2 = 49 \)

=> \(x^2 + 8xy+ (4)y^2 = 7^2 \)

=> \((x + 4y)^2 = 7^2\)

=> x + 4y = 7

=> x = 7 - 4y

Then - x = -7 + 4y or 4y - 7

Answer E
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BrentGMATPrepNow
Bunuel
If x is negative and y is positive, what is the value of x given that \(x^2−8xy+16y^2=49\) ?

A. 4y − 49
B. 49 − 4y
C. −7 − 4y
D. 7 + 4y
E. 4y − 7
GIVEN: x² − 8xy + 16y² = 49
Factor left side to get: (x - 4y)(x - 4y) = 49
Or we can write: (x - 4y)² = 49

If SOMETHING² = 49, then either SOMETHING = 7 or SOMETHING = -7
So, EITHER (x - 4y) = 7 OR (x - 4y) = -7

Since x is negative and y is positive, x - 4y = NEGATIVE - 4(POSITIVE) = some negative number
So, it must be the case that x - 4y = -7
Add 4y to both sides to get: x = -7 + 4y
This is the SAME as x = 4y - 7

Answer: E

Cheers,
Brent
­but how do we know |x| is greater than 4y
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Carolweszxdrcftvb
BrentGMATPrepNow
Bunuel
If x is negative and y is positive, what is the value of x given that \(x^2−8xy+16y^2=49\) ?

A. 4y − 49
B. 49 − 4y
C. −7 − 4y
D. 7 + 4y
E. 4y − 7
GIVEN: x² − 8xy + 16y² = 49
Factor left side to get: (x - 4y)(x - 4y) = 49
Or we can write: (x - 4y)² = 49

If SOMETHING² = 49, then either SOMETHING = 7 or SOMETHING = -7
So, EITHER (x - 4y) = 7 OR (x - 4y) = -7

Since x is negative and y is positive, x - 4y = NEGATIVE - 4(POSITIVE) = some negative number
So, it must be the case that x - 4y = -7
Add 4y to both sides to get: x = -7 + 4y
This is the SAME as x = 4y - 7

Answer: E

Cheers,
Brent
­but how do we know |x| is greater than 4y
­
From \((x - 4y)^2 = 49\), by taking the square root, we get \(|x - 4y| = 7\). Given that x is negative and y is positive, x - 4y = negative - positive = nehgative, thus \(|x - 4y| = -(x - 4y)\). Therefore, \(|x - 4y| = 7\) transforms into \(-(x - 4y) = 7\), which finally gives \(x = 4y - 7\).

Hope this helps.
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