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# If y^-2 + 2y^-1

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If y^-2 + 2y^-1 [#permalink]  02 Mar 2018, 16:29
Expert's post
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Question Stats:

50% (02:34) correct 50% (02:16) wrong based on 6 sessions
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. $$-5$$
[Reveal] Spoiler: OA

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Joined: 18 Apr 2015
Posts: 4864
Followers: 74

Kudos [?]: 965 [0], given: 4466

Re: If y^-2 + 2y^-1 [#permalink]  02 Mar 2018, 16:39
Expert's post
Explanation

$$\frac{1}{y^2} + \frac{1}{2y^1} -\frac{1}{15} = 0$$

This is like to write $$y^2 + 2y - 15 =0$$

Now, this is a quadratic equation, in which the multiplication is 15 and the sum is 2.

Two numbers that give us this are +5 and -3

y + 5 =0

y - 3 =0

OR

y = - 5
y = 3

At this point, the answer could be A and E but we do know that the answer must be one and only. Do not forget that we flipped the entire equation for simplicity. We should turn back because our y was raised to -1.

Which means that our 3 becomes $$\frac{1}{3}$$ and our -5 become $$\frac{- 1}{5}$$.

As you can see, we do have one and only correct solution among the answer choices that is $$\frac{- 1}{5}$$.

C is correct
_________________
Re: If y^-2 + 2y^-1   [#permalink] 02 Mar 2018, 16:39
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# If y^-2 + 2y^-1

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