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What is the sum of all solutions to the equation

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What is the sum of all solutions to the equation  [#permalink]

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New post 28 Feb 2017, 08:31
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Difficulty:

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Question Stats:

64% (02:28) correct 36% (02:40) wrong based on 166 sessions

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What is the sum of all solutions to the equation \(3x^{2/3} = 54 + 9x^{1/3}\)

A) 63
B) 189
C) 216
D) 243
E) 567

*kudos for all correct solutions

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What is the sum of all solutions to the equation  [#permalink]

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New post 01 Mar 2017, 05:46
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GMATPrepNow wrote:
What is the sum of all solutions to the equation \(3x^{2/3} = 54 + 9x^{1/3}\)

A) 63
B) 189
C) 216
D) 243
E) 567


We should start by recognizing that this is a QUADRATIC EQUATION in disguise.
Notice that \(x^{2/3} = (x^{1/3})^{2}\)
So let's let \(x^{1/3}\) = k, and replace \(x^{1/3}\) with k to get: 3k² = 54 + 9k
Rearrange to get: 3k² - 9k - 54 = 0
Factor: 3(k² - 3k - 18) = 0
Factor more: 3(k - 6)(k + 3) = 0
So, the solutions are k = 6 and k = -3

Since k = \(x^{1/3}\), we can write: \(x^{1/3}\) = 6 and \(x^{1/3}\) = -3

If \(x^{1/3}\) = 6, then x = 6³ = 216
If \(x^{1/3}\) = -3, then x = (-3)³ = -27

So, the sum of all solutions = 216 + (-27) = 189

Answer: B

Cheers,
Brent
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Re: What is the sum of all solutions to the equation  [#permalink]

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New post 28 Feb 2017, 08:38
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Let x^(1/3) = a

3a^2 = 54 + 9a
a^2 = 18 + 3a
a^2 - 3a - 18 = 0

Upon solving, we get a = 6 or -3
x^(1/3) = 6 --> x = 6^3 = 216
x^(1/3) = -3 --> x = -27

Sum of the solutions = 216 - 27 = 189

Answer: B
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Re: What is the sum of all solutions to the equation  [#permalink]

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New post 16 Jul 2017, 12:52
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What is the sum of all solutions to the equation \(3x^{2/3} = 54 + 9x^{1/3}\)

Let \(x^{1/3} = a\)

\(3a^2 = 54 + 9a\)

\(3a^2 - 9a - 54 = 0\)

\(a^2 - 3a - 18 = 0\)

\((a - 6) (a + 3) = 0\)

\(a = 6\) Or

\(a = -3\)


As, \(x^{1/3} = a\)

\(x^{1/3} = 6\)

\(x = 6 * 6 * 6 = 216\)

\(x^{1/3} = -3\)

\(x = -3 * -3 * -3 = -27\)

So, sum of the roots \(= 216 + (-27) = 216 - 27 = 189\)


Hence, Answer is B

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Re: What is the sum of all solutions to the equation  [#permalink]

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New post 26 Feb 2018, 19:05
GMATPrepNow wrote:
GMATPrepNow wrote:
What is the sum of all solutions to the equation \(3x^{2/3} = 54 + 9x^{1/3}\)

A) 63
B) 189
C) 216
D) 243
E) 567


We should start by recognizing that this is a QUADRATIC EQUATION in disguise.
Notice that \(x^{2/3} = (x^{1/3})^{2}\)
So let's let \(x^{1/3}\) = k, and replace \(x^{1/3}\) with k to get: 3k² = 54 + 9k
Rearrange to get: 3k² - 9k - 54 = 0
Factor: 3(k² - 3k - 18) = 0
Factor more: 3(k - 6)(k + 3) = 0
So, the solutions are k = 6 and k = -3

Since k = \(x^{1/3}\), we can write: \(x^{1/3}\) = 6 and \(x^{1/3}\) = -3

If \(x^{1/3}\) = 6, then x = 6³ = 216
If \(x^{1/3}\) = -3, then x = (-3)³ = -27

So, the sum of all solutions = 216 + (-27) = 189

Answer: B

Cheers,
Brent



thanks for the explanation - I got to the part where k = -6 or 3. Why are we not plugging these numbers back into the equation \(3x^{2/3} = 54 + 9x^{1/3}\) replacing x for -6 and 3?
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Re: What is the sum of all solutions to the equation &nbs [#permalink] 26 Feb 2018, 19:05
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