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Re: Two different positive numbers a and b each differ from their reciproc [#permalink]
Bunuel wrote:
Two different positive numbers a and b each differ from their reciprocals by 1. What is a + b ?


(A) 1
(B) 2
(C) \(\sqrt{5}\)
(D) \(\sqrt{6}\)
(E) 3


I am not sure whether such quadratic properties are tested on gmat
this question will yield an eqn x^2+x-1 and x^2+x+1
we need to use formula b^2+/-√4ac/2a
IMO C would be correct...
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Re: Two different positive numbers a and b each differ from their reciproc [#permalink]
Could anyone tell me why 1/b -b is taken as -1 since both the numbers differ from their reciprocals by 1 only

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Re: Two different positive numbers a and b each differ from their reciproc [#permalink]
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vishumangal wrote:
Could anyone tell me why 1/b -b is taken as -1 since both the numbers differ from their reciprocals by 1 only

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The question says both the numbers differ from their reciprocals which means \(\frac{1}{a} - a = 1\) and \(\frac{1}{b} - b = 1\) but since both the numbers are different that's why one of the number will differ from its reciprocal by \(b - \frac{1}{b} = 1\) or we can rewrite as \(\frac{1}{b} - b = -1\).
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Re: Two different positive numbers a and b each differ from their reciproc [#permalink]
Abhishek292kumar, hi can you please tell how did you come to the value of a as -1/2 + ((5)^(1/2))/2 ?
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Re: Two different positive numbers a and b each differ from their reciproc [#permalink]
abhishek31 wrote:
hi can you please tell how did you come to the value of a as -1/2 + ((5)^(1/2))/2 ?

Hi,
By using the formula of quadratic equation:
\(a{x^2} + bx + c = 0\) then \(x =\frac { -b + \sqrt{{b^2}-4ac}}{2a}\) and \(x =\frac { -b - \sqrt{{b^2}-4ac}}{2a}\)
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Two different positive numbers a and b each differ from their reciproc [#permalink]
Expert Reply
Bunuel wrote:
Two different positive numbers a and b each differ from their reciprocals by 1. What is a + b ?


(A) 1
(B) 2
(C) \(\sqrt{5}\)
(D) \(\sqrt{6}\)
(E) 3


3/2 - 2/3 = 5/6
5/3 - 3/5 = 16/15

The red option yields a difference that is a little LESS than 1.
The blue option yields a difference that is a bit MORE than 1.
To yield a difference of 1, we need a combination of values between the red option and the blue option.
Since the blue difference is closer to 1 than is the red difference, the larger of the two values must be just a bit less than 5/3.

Implication:
One of the values ≈ 5/3 (since it differs from its reciprocal of 3/5 by about 1)
The other value ≈ 3/5 (since it differs from its reciprocal of 5/3 by about 1)

Thus:
a + b ≈ 5/3 + 3/5 = 34/15 ≈ 30/15 + 4/15 ≈ 2 + 1/4 = 2.25
The closest answer choice is C:
\(\sqrt{5} ≈ 2.24\)

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Two different positive numbers a and b each differ from their reciproc [#permalink]
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