Math Help

This is a discussion on Math Help within the A Brief History of Cprogramming.com forums, part of the Community Boards category; e^x + x = 5 How do you solve this and what is the answer? It is a review question ...

  1. #1
    ¡Amo fútbol!
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    Math Help

    e^x + x = 5

    How do you solve this and what is the answer? It is a review question for a math quiz and I have tried many times to solve this. It will probably require natural logs because that is what I am doing currently.

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    Re: Math Help

    Originally posted by golfinguy4
    e^x + x = 5

    How do you solve this and what is the answer? It is a review question for a math quiz and I have tried many times to solve this. It will probably require natural logs because that is what I am doing currently.
    (e^x + x) - x = (5) - x

    e^x = 5 - x

    ln(e^x) = ln(5 - x)

    thus, x = ln(5 - x).

    i don't know if that's right though...

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    i forgot to mention that ln e^x = x.

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    or was it e^(ln x) = x?

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    geek SilentStrike's Avatar
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    e^(ln(x)) = ln(e^(x)) = x
    e^x != x
    Prove you can code in C++ or C# at TopCoder, referrer rrenaud
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  6. #6
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    I have gotten that far. However, x=ln(5-x) doesn't solve for x because there are x's on both sides of the equation.


    P.S. It is both

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    Just because ygfperson's Avatar
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    i got the value by writing a program to find values that fit the equation. i'm just as baffled as y'all are, but hopefully this will help

  8. #8
    Ethereal Raccoon Procyon's Avatar
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    I don't think that equation can be solved non-numerically. I have Mathematica, and it gave the answer as

    x = 5 - ProductLog[e^5]

    which doesn't say much more than the original equation. It numerically evaluates to

    1.3065586410395

    which is about as good an answer as you're going to get.

    If this is for a high school algebra class, I think your teacher goofed.

  9. #9
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    yfg, I can get that answer graphically by finding the intersection on my calc but I want to be able to find it algebraically b/c that is the way my teacher wants us to solve it.

    Procyon: What the heck is productlog?


    P.S. It is for a Pre-Calc class and from the book.

  10. #10
    Just because ygfperson's Avatar
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    i'm in pre-calc too, and i've never seen that kind of thing before. i think the book made a mistake or something

  11. #11
    Ethereal Raccoon Procyon's Avatar
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    Originally posted by golfinguy4
    Procyon: What the heck is productlog?
    According to Mathematica:

    ProductLog[z] satisfies the differential equation dw/dz = w/(z(1+w)).

    It's approximately equal to x - x^2 + 3/2 x^3 - 8/3 x^4.

    There's apparently no way to solve this algebraically. I think your book is probably in error.

  12. #12
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    I saw my teacher today. She told me that it couldn't be done algebraically and the book was only wanting me to do it graphically. I assumed that any problem could be solved more than 1 way, guess I was wrong.

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