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Must see for Maximum/Minimum value problems

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Shalabh's Quants Really wants to Beat The GMAT!
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Must see for Maximum/Minimum value problems Post Sun Apr 08, 2012 10:02 am
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  • Lap #[LAPCOUNT] ([LAPTIME])
    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.

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    Post Sun Apr 08, 2012 10:17 am


    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.

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    Post Sun Apr 08, 2012 10:33 am
    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.

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    Post Sun Apr 08, 2012 10:45 am
    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.

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    Post Sun Apr 08, 2012 11:15 am
    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.

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    sureshs Just gettin' started! Default Avatar
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    Post Thu Apr 12, 2012 5:04 am
    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.
    Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.

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    Post Fri Apr 13, 2012 6:46 pm
    hi Shalabh

    Good tips. Can you tell us where did you get these questions from? Are these from OG?

    Thanks

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    Post Fri Apr 13, 2012 9:58 pm
    iwillsurvive101 wrote:
    hi Shalabh

    Good tips. Can you tell us where did you get these questions from? Are these from OG?

    Thanks
    Thanks! These are compilations from various GMAT TestPrep cos..Not from OG though.

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    Post Fri Apr 13, 2012 11:46 pm
    sureshs wrote:
    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.
    Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.
    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.

    _________________
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    e-GMAT Instructor

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    sureshs Just gettin' started! Default Avatar
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    Post Sat Apr 14, 2012 5:38 am
    Shalabh's Quants wrote:
    sureshs wrote:
    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.
    Can anyone explain...Why to maximise 3x.4y? We want to maximise xy.
    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.
    Ok Ok. I got it sir. thank you v. much.

    satyavegi Just gettin' started! Default Avatar
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    Post Fri May 18, 2012 2:44 am
    Hi Shalabh

    Can You also explain where all we can use this concept in detail

    regards
    Vegi

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