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If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f

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Joined: 02 Sep 2009
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If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f  [#permalink]

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06 Feb 2020, 04:30
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If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

(A) 1
(B) k
(C) 7k - 1
(D) k^2 + k
(E) k^2 - k

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Joined: 14 Dec 2019
Posts: 494
Location: Poland
GMAT 1: 570 Q41 V27
WE: Engineering (Consumer Electronics)
Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f  [#permalink]

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06 Feb 2020, 05:01
If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

(A) 1
(B) k
(C) 7k - 1
(D) k^2 + k
(E) k^2 - k

f(4) = k(4-k); f(3) = k(3-k)

f(4) - f(3) = 4k-$$k^2$$-3k+$$k^2$$ -> k - Answer - B
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Joined: 11 Sep 2015
Posts: 4579
GMAT 1: 770 Q49 V46
Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f  [#permalink]

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06 Feb 2020, 07:20
Top Contributor
Bunuel wrote:
If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

(A) 1
(B) k
(C) 7k - 1
(D) k^2 + k
(E) k^2 - k

Given: f(x) = k(x - k)
So, for example, f(9) = k(9 - k) = 9k - k²

The question asks for the value of f(4) - f(3)
f(4) - f(3) = [k(4 - k)] - [k(3 - k)]
= [4k - k²] - [3k - k²]
= k

Cheers,
Brent
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Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f  [#permalink]

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06 Feb 2020, 07:26
Bunuel wrote:
If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

(A) 1
(B) k
(C) 7k - 1
(D) k^2 + k
(E) k^2 - k

f(4) = k(4-k)
f(3) = k(3-k)
f(4) - f(3) = k

IMO B
Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f   [#permalink] 06 Feb 2020, 07:26
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