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D01-41

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Manager
Joined: 06 Sep 2018
Posts: 176
Location: Pakistan
Concentration: Finance, Operations
GPA: 2.87
WE: Engineering (Other)

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14 Oct 2018, 20:54
Bunuel wrote:
The equation $$x^2 + ax - b = 0$$ has equal roots, and one of the roots of the equation $$x^2 + ax + 15 = 0$$ is 3. What is the value of b?

A. $$-\frac{1}{64}$$
B. $$-\frac{1}{16}$$
C. $$-15$$
D. $$-16$$
E. $$-64$$

as $$x=3$$ for $$x^2 + ax + 15 = 0$$
So $$9+3a+15=0$$
$$a=-8$$
put value of a in $$x^2 + ax - b = 0$$
So $$x^2 - 8x - b = 0$$
As roots are equal so sum of roots is $$x+x=\frac{-(-8)}{1}$$
$$2x=8$$
$$x=4$$
now put $$x=4$$ in $$x^2 - 8x - b = 0$$
So $$16-8(4)-b=0$$
$$16-32-b=0$$
$$b=-16$$
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Intern
Joined: 13 Nov 2018
Posts: 26
Location: United States
Schools: Tepper '21 (I)
GMAT 1: 650 Q39 V40
GMAT 2: 660 Q44 V37
GPA: 2.99

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26 Nov 2018, 11:24
I'm Getting tripped up by the verbiage. Equal roots to me means x^2+ax−b=0 can have roots of 2 and 2, 4 and 4 etc.

I'm stuck at how you chose the Discriminate to be 0 instead of positive.

some clarity would be very helpful
Intern
Joined: 18 Jan 2018
Posts: 19
Location: India
Concentration: Finance, Marketing
GPA: 3.98

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18 Jan 2019, 23:22
I think this is a high-quality question and the explanation isn't clear enough, please elaborate.
Math Expert
Joined: 02 Sep 2009
Posts: 52935

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18 Jan 2019, 23:52
aalakshaya wrote:
I think this is a high-quality question and the explanation isn't clear enough, please elaborate.

Could you please ask a specific question? There are three pages of discussion of this question. Not sure what to elaborate more. Thank you.
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Re: D01-41   [#permalink] 18 Jan 2019, 23:52

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