GRE Prep Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Given Kudos: 136
If f(2x - 1) = 4x² - 4x - 8, then f(5) =
[#permalink]
17 Apr 2019, 09:50
17 Apr 2019, 09:50
2
00:00
Question Stats:
95% (00:43) correct
5% (01:04) wrong based on 40 sessions
Hide Show timer Statistics
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Given Kudos: 136
Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =
[#permalink]
19 Apr 2019, 04:35
19 Apr 2019, 04:35
1
GreenlightTestPrep wrote:
Approach #1:
Solve the equation 2x - 1 = 5 to get x = 3
Take: f(2x - 1) = 4x² - 4x - 8
Plug in x = 3 to get: f(2(3) - 1) = 4(3)² - 4(3) - 8
Simplify: f(5) = 36 - 12 - 8 = 16
Answer: 16
Cheers,
Brent
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Given Kudos: 136
Re: If f(2x - 1) = 4x² - 4x - 8, then f(5) =
[#permalink]
19 Apr 2019, 04:36
19 Apr 2019, 04:36
1
GreenlightTestPrep wrote:
Approach #2:
First notice that 4x² - 4x - 8 looks a lot like (2x - 1)²
(2x - 1)² = (2x - 1)(2x - 1) = 4x² - 4x + 1
Now take: f(2x - 1) = 4x² - 4x - 8
Rewrite as: f(2x - 1) = 4x² - 4x + 1 - 9 = (2x - 1)² - 9
In other words, f(something) =something² - 9
In general, we can write f(x) = x² - 9
We're asked to evaluate f(5)
So f(5) = 5² - 9 = 25 - 9 = 16
Answer: 16
Cheers,
Brent
