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# What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0

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Math Expert
Joined: 02 Sep 2009
Posts: 50004
What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0  [#permalink]

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23 Sep 2018, 21:52
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Difficulty:

15% (low)

Question Stats:

82% (00:52) correct 18% (01:12) wrong based on 43 sessions

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What is the value of 5x^2 + 4x - 1 ?

(1) x(x + 2) = 0
(2) x = 0

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ISB, NUS, NTU Moderator
Joined: 11 Aug 2016
Posts: 265
Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0  [#permalink]

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23 Sep 2018, 22:34
Bunuel wrote:
What is the value of 5x^2 + 4x - 1 ?

(1) x(x + 2) = 0
(2) x = 0

$$5x^2+4x-1$$
$$=5x^2+5x-x-1$$
$$=5x(x+1)-1(x+1)$$
$$=(x+1)(5x-1)$$

Statement 1: x(x + 2) = 0
Either x=0 or x=-2
If x=0,
$$5x^2+4x-1=(+1)(-1)=-1$$

If x=-2
$$5x^2+4x-1=(-2+1)(-10-1)=(-1)(-11)=11$$
Insufficient

Statement 2: x = 0
It will lead to a single value of expression as seen above.
Sufficient.

NOTE: In the case of first statement, it is absolutely necessary to check the value of the expression for any two given values of x.
Since quadratics represent a parabola on a XY plane, there will be 2 values of X for any given value of the quadratic expression(except the lowest or the highest value).
Question makers can trick test takers into this trap.

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Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 &nbs [#permalink] 23 Sep 2018, 22:34
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