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    the sum of two numbers x and y is constant and equal to 5

    a) write down an expression for the product p of the two numbers in terms of x
    b)hence find the values of x and y which maximise the product

    any ideas?
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    Show YOUR working. We already know how to do it.
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    (Original post by Mr M)
    Show YOUR working. We already know how to do it.
    i don't have any working to show, our else I would


    i am unsure of where to start
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    (Original post by madfish)
    i don't have any working to show, our else I would


    i am unsure of where to start
    x+y=5
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    (Original post by madfish)
    i don't have any working to show, our else I would


    i am unsure of where to start
    Write down the sum you are given in terms of x and y.

    Make y the subject.

    Write down the product you are given in terms of x and y.

    Eliminate y in the product.

    Report back when you have done this.
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    (Original post by Mr M)
    Write down the sum you are given in terms of x and y.

    Make y the subject.

    Write down the product you are given in terms of x and y.

    Eliminate y in the product.

    Report back when you have done this.
    done, now what?
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    (Original post by madfish)
    done, now what?
    You have a function in x that you wish to maximise

    What do you do to find the maximum of a function
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    (Original post by madfish)
    done, now what?
    Provide your answer so far.

    You need to differentiate now.
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    (Original post by TenOfThem)
    You have a function in x that you wish to maximise

    What do you do to find the maximum of a function
    I get x+y=5 then p=xy

    then p/x=5-x

    not sure what to do now
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    (Original post by madfish)
    I get x+y=5 then p=xy

    then p/x=5-x

    not sure what to do now
    Multiply through by x.

    Now find \frac{dP}{dx}
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    (Original post by Mr M)
    Multiply through by x.

    Now find \frac{dP}{dx}
    okay so DP/DX = 5-2x then x=5/2
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    (Original post by madfish)
    okay so dP/dx = 5-2x then x=5/2
    Yes and what does y equal?
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    (Original post by Mr M)
    Yes and what does y equal?
    I guess I sub in x to the original equation to get y?

    y=0.5?
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    (Original post by madfish)
    I guess I sub in x to the original equation to get y?

    y=0.5?
    2.5 + 0.5 \neq 5
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    (Original post by Mr M)
    2.5 + 0.5 \neq 5
    x=y=2/5
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    (Original post by madfish)
    x=y=2/5
    0.4 + 0.4 \neq 5
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    (Original post by Mr M)
    0.4 + 0.4 \neq 5
    5/2!! damn sorry copied 2/5 down by mistake
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    (Original post by madfish)
    5/2!! damn sorry copied 2/5 down by mistake
    Yes that's right.
 
 
 
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