Hi,
There are few problems for Maximum/Minimum value deduction that are tricky and time consuming. Pl. see below few examples of those with Take Away approach
Q.What is the Maximum value of =(7-x)^5 (7+x)^4?
A.7^9
B.14^4
C.0
D.7^9.2^17.5^5/3^18
E.7^9.2^7.5^7/3^18
------------------------------------------------
Take Away...
If a,b, c are variables and a+b+c is constant,
& Let X=a^p.b^q.c^r
then for x to be maximum...a/p=b/q=c/r
In above problem, say x=(7-x)^5 (7+x)^4, where a=(7-x); b=(7+x); p=5; q=4.
As a+b=(7-x)+(7+x)=14 (Constant), hence to get max of X...
=> Do (7-x)/5=(7+x)/4
=>which yields...x=-7/9
=> by putting the value of x=-7/9 in (7-x)^5 (7+x)^4, we get 7^9.2^17.5^5/3^18.
Answer is D.
Must see for Maximum/Minimum value problems
This topic has expert replies
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Q.What is the minimum value of x+1/x ? If x>0.
A.2
B.1
C.1/2
D.sqrt 2
E.3/2
--------------------------------------------
Take Away..
If A*B=constant; where A & B are variables,
then A+B=Minimum, when A=B.
Since in given problem x+1/x,
x*1/x= 1 (Constant), hence x+1/x will be minimum when x= 1/x
=> so, x= 1/x => x^2=1 => x= 1, -1.
=> Take x=1 only as x>0.
Hence Minimum value of x+1//x=2.
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Q.What is the maximum value of xy ? If 3x+4Y=24.
A.24
B.12
C.120
D.0
E.infinity
--------------------------------------------
Take Away..
If A+B=constant; where A & B are variables,
then A*B=maximum, when A=B.
We can write xy as 3x.4y/12.
As 3x+4Y=24,
so for 3x.4y to be max 3x=4y=24/2=12.
This gives x=4, y=3
The maximum value of xy would be 4.3=12.
A.24
B.12
C.120
D.0
E.infinity
--------------------------------------------
Take Away..
If A+B=constant; where A & B are variables,
then A*B=maximum, when A=B.
We can write xy as 3x.4y/12.
As 3x+4Y=24,
so for 3x.4y to be max 3x=4y=24/2=12.
This gives x=4, y=3
The maximum value of xy would be 4.3=12.
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Q.What is the minimum value of 1/x+1/y+1/z ? If x+y+z=1.
A.1
B.3
C.6
D.8
E.9
--------------------------------------------
Take Away..
If A+B+C=constant; where A, B & C are variables,
then A*B*C=maximum, when A=B=C.
=> 1/x+1/y+1/z = (xy+yz+xz)/xyz;
=> for (xy+yz+xz)/xyz to be minimum; denominator xyz should be maximum.
=> since x+y+z=1, hence at x=y=z, xyz will be max.
=> this gives x=y=z=1/3,
or, 1/x+1/y+1/z= 9.
A.1
B.3
C.6
D.8
E.9
--------------------------------------------
Take Away..
If A+B+C=constant; where A, B & C are variables,
then A*B*C=maximum, when A=B=C.
=> 1/x+1/y+1/z = (xy+yz+xz)/xyz;
=> for (xy+yz+xz)/xyz to be minimum; denominator xyz should be maximum.
=> since x+y+z=1, hence at x=y=z, xyz will be max.
=> this gives x=y=z=1/3,
or, 1/x+1/y+1/z= 9.
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Q. What is the minimum possible value of f(x)=max {(3-x), (2+x), (5-x)} ? Where X is an integer.
A.5
B.6
C.0
D.3
E.4
---------------------------------------
We need to get Minimum of [f(x)= max {(3-x), (2+x), (5-x)}]
Check for 0, +ive and -ive values of x and see the behaviour of fn.
By Putting x=0, f(0)= max {(3-0), (2+0), (5-0)}= max {3, 2, 5} = 5.
Similarly...
=> x=0, f(x)=5,
=> x=-1, f(x)=6,
=> x=1, f(x)=4,
=> x=2, f(x)=4,
=> x=3, f(x)=5,
As we take higher +ive values, f(x) increase so is the case with -ive values.
Hence f(x) will have minimum value as 4.
A.5
B.6
C.0
D.3
E.4
---------------------------------------
We need to get Minimum of [f(x)= max {(3-x), (2+x), (5-x)}]
Check for 0, +ive and -ive values of x and see the behaviour of fn.
By Putting x=0, f(0)= max {(3-0), (2+0), (5-0)}= max {3, 2, 5} = 5.
Similarly...
=> x=0, f(x)=5,
=> x=-1, f(x)=6,
=> x=1, f(x)=4,
=> x=2, f(x)=4,
=> x=3, f(x)=5,
As we take higher +ive values, f(x) increase so is the case with -ive values.
Hence f(x) will have minimum value as 4.
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.Shalabh's Quants wrote:Q.What is the maximum value of xy ? If 3x+4Y=24.
A.24
B.12
C.120
D.0
E.infinity
--------------------------------------------
Take Away..
If A+B=constant; where A & B are variables,
then A*B=maximum, when A=B.
We can write xy as 3x.4y/12.
As 3x+4Y=24,
so for 3x.4y to be max 3x=4y=24/2=12.
This gives x=4, y=3
The maximum value of xy would be 4.3=12.
- iwillsurvive101
- Senior | Next Rank: 100 Posts
- Posts: 58
- Joined: Mon Nov 07, 2011 7:44 pm
- Location: Washington DC
- Thanked: 1 times
hi Shalabh
Good tips. Can you tell us where did you get these questions from? Are these from OG?
Thanks
Good tips. Can you tell us where did you get these questions from? Are these from OG?
Thanks
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Thanks! These are compilations from various GMAT TestPrep cos..Not from OG though.iwillsurvive101 wrote:hi Shalabh
Good tips. Can you tell us where did you get these questions from? Are these from OG?
Thanks
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
- Shalabh's Quants
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Fri Apr 06, 2012 3:11 am
- Thanked: 35 times
- Followed by:5 members
Since given is 3x+4Y=24 = constant. We can maximise the product of elements present in 3x+4Y=24 only i.e. 3x and 4y. By way of maximising 3x.4y = 12xy, we can also deduce maximim value of xy.sureshs wrote:Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.Shalabh's Quants wrote:Q.What is the maximum value of xy ? If 3x+4Y=24.
A.24
B.12
C.120
D.0
E.infinity
--------------------------------------------
Take Away..
If A+B=constant; where A & B are variables,
then A*B=maximum, when A=B.
We can write xy as 3x.4y/12.
As 3x+4Y=24,
so for 3x.4y to be max 3x=4y=24/2=12.
This gives x=4, y=3
The maximum value of xy would be 4.3=12.
Shalabh Jain,
e-GMAT Instructor
e-GMAT Instructor
Ok Ok. I got it sir. thank you v. much.Shalabh's Quants wrote:Since given is 3x+4Y=24 = constant. We can maximise the product of elements present in 3x+4Y=24 only i.e. 3x and 4y. By way of maximising 3x.4y = 12xy, we can also deduce maximim value of xy.sureshs wrote:Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.Shalabh's Quants wrote:Q.What is the maximum value of xy ? If 3x+4Y=24.
A.24
B.12
C.120
D.0
E.infinity
--------------------------------------------
Take Away..
If A+B=constant; where A & B are variables,
then A*B=maximum, when A=B.
We can write xy as 3x.4y/12.
As 3x+4Y=24,
so for 3x.4y to be max 3x=4y=24/2=12.
This gives x=4, y=3
The maximum value of xy would be 4.3=12.