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D01-41

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Joined: 06 Sep 2018
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Concentration: Finance, Operations
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Re: D01-41  [#permalink]

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New post 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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Re: D01-41  [#permalink]

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New post 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
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Re D01-41  [#permalink]

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New post 18 Jan 2019, 23:22
I think this is a high-quality question and the explanation isn't clear enough, please elaborate.
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Re: D01-41  [#permalink]

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New post 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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