If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k? - The Beat The GMAT Forum

If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

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BTGModeratorVI: Legendary Member; Posts: 1223; Joined: Sat Feb 15, 2020 2:23 pmFollowed by: 1 member

by BTGModeratorVI » Fri May 29, 2020 6:39 am

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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

Answer: B
Source: GMAT Hacks

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Source: Beat The GMAT — Problem Solving | Fri May 29, 2020 6:39 am

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

by Brent@GMATPrepNow » Tue Jun 02, 2020 5:44 am

BTGModeratorVI wrote: ↑
Fri May 29, 2020 6:39 am
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

Answer: B
Source: GMAT Hacks

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

Answer: B

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8176; Joined: Sat Apr 25, 2015 10:56 amFollowed by: 29 members; Location: Los Angeles, CA; Thanked: 43 times

Re: If f(x) = k(x - k) and k is a constant, what is the value of f(4) - f(3), in terms of k?

by Scott@TargetTestPrep » Sat Jun 06, 2020 5:49 am

BTGModeratorVI wrote: ↑
Fri May 29, 2020 6:39 am
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

Answer: B

Solution:

f(4) = k(4 - k) = 4k - k^2

f(3) = k(3 - k) = 3k - k^2

So, we have:

f(4) - f(3) = 4k - k^2 - (3k - k^2) = 4k - k^2 - 3k + k^2 = k

Answer: B

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