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# If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu

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Math Expert
Joined: 02 Sep 2009
Posts: 47967
If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu  [#permalink]

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05 Jul 2018, 21:28
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Difficulty:

35% (medium)

Question Stats:

68% (01:38) correct 32% (01:58) wrong based on 68 sessions

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If $$y^{-2} + 2y^{-1} -15 = 0$$, which of the following could be the value of y?

A. $$3$$

B. $$\frac{1}{5}$$

C. $$\frac{-1}{5}$$

D. $$\frac{-1}{3}$$

E. $$-5$$

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Senior Manager
Status: Learning stage
Joined: 01 Oct 2017
Posts: 491
WE: Supply Chain Management (Energy and Utilities)
If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu  [#permalink]

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05 Jul 2018, 21:38
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Bunuel wrote:
If $$y^{-2} + 2y^{-1} -15 = 0$$, which of the following could be the value of y?

A. $$3$$

B. $$\frac{1}{5}$$

C. $$\frac{-1}{5}$$

D. $$\frac{-1}{3}$$

E. $$-5$$

Let $$y^{-1}=x$$

So, $$y^{-2} + 2y^{-1} -15 = 0$$
Or, $$x^2+2x-15=0$$
Or, x=-5 or x=3

1. x=-5, or, $$y^{-1}=-5$$ or, $$y=\frac{-1}{5}$$
2. x=3,or, $$y^{-1}=3$$ or, $$y=\frac{1}{3}$$

Among given options, correct answer is (C)
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PKN

Rise above the storm, you will find the sunshine

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Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu  [#permalink]

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09 Jul 2018, 03:44
PKN wrote:
Bunuel wrote:
If $$y^{-2} + 2y^{-1} -15 = 0$$, which of the following could be the value of y?

A. $$3$$

B. $$\frac{1}{5}$$

C. $$\frac{-1}{5}$$

D. $$\frac{-1}{3}$$

E. $$-5$$

Let $$y^{-1}=x$$

So, $$y^{-2} + 2y^{-1} -15 = 0$$
Or, $$x^2+2x-15=0$$
Or, x=-5 or x=3

1. x=-5, or, $$y^{-1}=-5$$ or, $$y=\frac{-1}{5}$$
2. x=3,or, $$y^{-1}=3$$ or, $$y=\frac{1}{3}$$

Among given options, correct answer is (C)

Is there an alternative way to solve this problem?
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Seryozha Sargsyan 21

Contact: sargsyanseryozha@gmail.com

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What you feel, you attract,
What you imagine, you create.

Senior Manager
Status: Learning stage
Joined: 01 Oct 2017
Posts: 491
WE: Supply Chain Management (Energy and Utilities)
Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu  [#permalink]

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09 Jul 2018, 03:51
Plugin answer choices and put in LHS of the given equation.The value of y for which the LHS becomes zero is the correct answer choice.

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Regards,

PKN

Rise above the storm, you will find the sunshine

Re: If y^(-2) + 2y^(-1) - 15 = 0, which of the following could be the valu &nbs [#permalink] 09 Jul 2018, 03:51
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