What is the minimum value of
f(x) = -5 + (x + 7)^2, and at what value
of x does it occur?
I solved above question with below approach.
- The minimum value for f(x) will be 0 and for that we would need (X+7)^2 = 5.
- So, I thought solving the equation x^2 + 14x + (49-5)=0 will give me the required values of x.
Apparently, I am wrong and the right answers are -7, -5.
Can you explain where have I gone wrong?
Thanking you!
Function problem
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 415
- Joined: Thu Oct 15, 2009 11:52 am
- Thanked: 27 times
Why does the minimum value of f(x) have to equal 0 ? Can't it be less than 0 ?baalok88 wrote: - The minimum value for f(x) will be 0
Looking at (x+7)^2 you can see it can only be positive so you want to keep the result as low as possible in order to minimize f(x). Can it = 0?
What if you made x equal to -7. That would make it 0. Then f(x) would be -5.
Your mistake was starting with an unexamined assumption that f(x) cannot be less than 0
Last edited by regor60 on Tue Oct 11, 2016 9:19 am, edited 1 time in total.
GMAT/MBA Expert
- [email protected]
- Elite Legendary Member
- Posts: 10392
- Joined: Sun Jun 23, 2013 6:38 pm
- Location: Palo Alto, CA
- Thanked: 2867 times
- Followed by:511 members
- GMAT Score:800
Hi baalok88,
This function is the sum of two terms... -5 and (X+7)^2. To find the smallest total, you have to think about how small you can make each of the two terms. The "-5" is locked in, but the value of (X+7)^2 will vary depending on the value of X. The smallest that you can make that term is 0 (when X = -7). Knowing that... when X = - 7, then the sum of the two terms is -5 + 0 = -5.
GMAT assassins aren't born, they're made,
Rich
This function is the sum of two terms... -5 and (X+7)^2. To find the smallest total, you have to think about how small you can make each of the two terms. The "-5" is locked in, but the value of (X+7)^2 will vary depending on the value of X. The smallest that you can make that term is 0 (when X = -7). Knowing that... when X = - 7, then the sum of the two terms is -5 + 0 = -5.
GMAT assassins aren't born, they're made,
Rich
GMAT/MBA Expert
- Brent@GMATPrepNow
- GMAT Instructor
- Posts: 16207
- Joined: Mon Dec 08, 2008 6:26 pm
- Location: Vancouver, BC
- Thanked: 5254 times
- Followed by:1268 members
- GMAT Score:770
As you can imagine, if this were are true GMAT question (with 5 answer choices), you could easily plug in the answer choices and determine the correct values.baalok88 wrote:What is the minimum value of f(x) = -5 + (x + 7)^2, and at what value of x does it occur?
Cheers,
Brent
-
- GMAT Instructor
- Posts: 2630
- Joined: Wed Sep 12, 2012 3:32 pm
- Location: East Bay all the way
- Thanked: 625 times
- Followed by:119 members
- GMAT Score:780
We can write any quadratic as
ax² + bx + c
If a > 0, the quadratic opens up. So the minimum will come when the two roots are the same. (If you visualize the quadratic, you get the minimum where there isn't anything to mirror the point at which you're at.)
Since the roots are of the form
x = (-b ± √(b² - 4ac))/2a
They'll be equal when
x = (-b + √(b² - 4ac))/2a = (-b - √(b² - 4ac))/2a
or when x = -b/2a.
Since your quadratic is
x² + 14x - 44 = 0
The minimum comes when x = -14/2 = -7. Then, plugging in x = -7, we get -5 + (-7 + 7)², or -5.
ax² + bx + c
If a > 0, the quadratic opens up. So the minimum will come when the two roots are the same. (If you visualize the quadratic, you get the minimum where there isn't anything to mirror the point at which you're at.)
Since the roots are of the form
x = (-b ± √(b² - 4ac))/2a
They'll be equal when
x = (-b + √(b² - 4ac))/2a = (-b - √(b² - 4ac))/2a
or when x = -b/2a.
Since your quadratic is
x² + 14x - 44 = 0
The minimum comes when x = -14/2 = -7. Then, plugging in x = -7, we get -5 + (-7 + 7)², or -5.
-
- GMAT Instructor
- Posts: 2630
- Joined: Wed Sep 12, 2012 3:32 pm
- Location: East Bay all the way
- Thanked: 625 times
- Followed by:119 members
- GMAT Score:780
You're on the right track, but you're thinking of a quadratic that already equals 0. This is an easy mistake to make, since the first introduction to quadratics in algebra (and for many people, the ONLY quadratics they worked with!) are when quadratic = 0.baalok88 wrote: I solved above question with below approach.
- The minimum value for f(x) will be 0 and for that we would need (X+7)^2 = 5.
- So, I thought solving the equation x^2 + 14x + (49-5)=0 will give me the required values of x.
But it isn't a law that the quadratic = 0: it could be more or less than that.
As for the second part, you were solving for the two ROOTS of the quadratic, another really common chore in high school algebra. Those are helpful, but they don't necessarily give you the minimums, only the point (if any) where the parabola (which is the name for the visualization of the quadratic) crosses the x-axis. (There are also parabolas that cross the y-axis in two places, but those don't tend to show up on the GMAT, so we won't bother with them.)
For much more on this, check out Purple Math.