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Math Help - Urgent help please

  1. #1
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    Urgent help please

    How can i write a system between 2 derivates with the ti-89?

    I mean

    d/dx [f(x,y)=a]
    where
    dy/dx=c

    for example if f(x,y)=a is x^2+3 =y

    then the solution is 2x=c => x=c/2



    I wasn't able to find anything... i've tried to do something like

    solve(d(x^2+3=y,x)|(d(y,x)=1),x) -->wrong value


    d(x^2+3=y,x)|(d(y,x)=1)) ------->2x=0 wrong


    Thanks in advance

    Marco
    Last edited by swatt; June 29th 2008 at 05:14 AM.
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  2. #2
    Moo
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    Hello,

    Instead of using |, separate your equations with and :

    Code:
    solve(d(x^2+3,x)=y and d(y,x)=-1,x)
    The problem is that they rather consider y as a variable than a function. This is partly why there is an error.

    There are huge problems in your syntax. For systems of equations, you must have : solve(...=!!! and :::=^^^,{variable1,variable2})

    After thinking about it, I don't really know...
    Last edited by Moo; June 29th 2008 at 02:05 AM.
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  3. #3
    swa
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    that equation give me a "false", maybe i was a bit unclear (i'm not english / american)..

    I have this equation:

    x^2+3 =y ->d/dx-> 2x=dy/dx

    now i impose dy/dx=c (it's the same thing i have a system of 2 equations)


    then the equation become 2x=c => x=c/2

    I think there should be a way to tell to the ti-89 to leave d/dx when it find derivates on unknown quantity/costant...
    Last edited by swa; June 29th 2008 at 05:17 AM.
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  4. #4
    swa
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    I've find a way, but it's not right at all..

    d(x^2+3=y|x=cy) -> 2x=c

    but in this case i think calc. will calculate the system firstly, and after it will do the derivate... so this is wrong too
    Last edited by swa; June 29th 2008 at 05:11 AM.
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  5. #5
    swa
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    I've found the solution.

    d(x^2+3=f(x),x) -> 2x=df(x)/dx

    then by hands i write df(x)/dx=c so x=c/2.

    Well, in this way it seems completely useless, but with giant eq. i usually resolve, it's a pretty nice way.
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