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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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Bunuel
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What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink] New post  23 Sep 2018, 20:52
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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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Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink] New post  23 Sep 2018, 21:34
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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.

Answer: B

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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mashrur amin
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Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink] New post  04 Nov 2019, 11:58
How do we get x=0 from statement 1? Didn’t understand.

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What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink] New post  04 Nov 2019, 12:23
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mashrur amin wrote:
How do we get x=0 from statement 1? Didn’t understand.

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Statement 1: x(x + 2) = 0

For this to be true either x or (x+2) has to be 0
because 0 multiplied by any no. will give 0

If x=0 Statement becomes 0(0+2)= 0
If x=-2 Statement becomes -2(-2+2)= 0

Hope it helps.
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Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink] New post  30 Dec 2020, 22:13
What is the value of 5x^2 + 4x - 1 ?

(1) x(x + 2) = 0
We have two values of x 0 and -2.
We would get 2 values for the equation.
Insufficient.

(2) x = 0
We are directly provided for the value of x = 0
We get only one value for the equation.
Sufficient.

Note: We might in hurry give C as the solution as after combining we also get x = 0; but careful as statement 2 provides us the same information without combining. This is an example of C trap.
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Re: What is the value of 5x^2 + 4x - 1 ? (1) x(x + 2) = 0 (2) x = 0 [#permalink]
30 Dec 2020, 22:13
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