Results 1 to 13 of 13

Math Help - General question about e^x

  1. #1
    Banned
    Joined
    Sep 2009
    Posts
    21

    General question about e^x

    Why does e^x - e^x=e^x?
    and why does e^x + e^x = e^x?

    This doesn't make sense. I know that the derivative of e^x=e^x, that makes sense. But the adding and subtracting 2 of them to = the same thing is weird.

    There are several examples in my book that show this. Please explain?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Banned
    Joined
    May 2009
    Posts
    471
    Quote Originally Posted by theredqueentheory View Post
    Why does e^x - e^x=e^x?
    and why does e^x + e^x = e^x?

    This doesn't make sense. I know that the derivative of e^x=e^x, that makes sense. But the adding and subtracting 2 of them to = the same thing is weird.

    There are several examples in my book that show this. Please explain?

    I would highly advise looking in your book one more time, because what you've written simply isn't true,

    y-y=0 for any and all y, including y=e^x

    y+y=2y for any and all y, including y=e^x
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Banned
    Joined
    Sep 2009
    Posts
    21

    thanks but....

    I could be wrong but could several other websites be wrong? I suppose they could...please see this about the problem:

    Derivatives of exponential and logarithmic functions - An approach to calculus

    "The derivative of ex with respect to x
    is equal to ex."
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Banned
    Joined
    May 2009
    Posts
    471
    I am fully aware that the derivative of e^x=e^x
    and that's what makes it special, but you wrote

    e^x+e^x=e^x

    and

    e^x-e^x=e^x which is nonsense
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Sep 2009
    Posts
    7
    derivative of e^x = e^x

    and i looked whole pages you stated
    and found nothing about

    e^x - e^x = e^x
    e^x + e^x = e^x

    which doesn't make sense to me...?

    since e^1 = 2.718281828
    and in (e^x - e^x) here,
    x = x
    so
    e^x - e^x = 0


    e^x + e^x = 2e^x i guess
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Banned
    Joined
    Sep 2009
    Posts
    21

    yes, nonsense.

    I agree, it is nonsense. Which is why I don't understand it.
    For example, a problem has (2+3)e^-3x + (2+3)e^-3x = 10^e-3x.

    Only one ^e-3x. So why when they add two of them does it only equal one?
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by theredqueentheory View Post
    I agree, it is nonsense. Which is why I don't understand it.
    For example, a problem has (2+3)e^-3x + (2+3)e^-3x = 10^e-3x.

    Only one ^e-3x. So why when they add two of them does it only equal one?
    2 + 3 = 5. So you have (5 + 5) = 10 lots of e^{-3x}.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Banned
    Joined
    Sep 2009
    Posts
    21

    um...

    so are you saying that e^{-3x} is used as a suffix in this case, and even though the 5 and 5 are added together (thanks for helping with that sum LOL) the e^{-3x} is not added?
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by theredqueentheory View Post
    so are you saying that e^{-3x} is used as a suffix in this case, and even though the 5 and 5 are added together (thanks for helping with that sum LOL) the e^{-3x} is not added?
    Let A represent e^{-3x}. It's very obvious that 5A + 5A = 10A.
    Follow Math Help Forum on Facebook and Google+

  10. #10
    Banned
    Joined
    Sep 2009
    Posts
    21

    ok...

    It may be obvious to super smart folks like you, I'm just a lowly biotechnologist and my current job in cancer research doesn't require calculus so I am resigned to being referred to as dumb. Condescend away!!! I don't mind at all.
    Thank you for elucidating that you do not add the suffix together. Then why was everyone in the posts above telling me that if you add two of them together it does not equal one?
    Follow Math Help Forum on Facebook and Google+

  11. #11
    Banned
    Joined
    May 2009
    Posts
    471
    Quote Originally Posted by theredqueentheory View Post
    It may be obvious to super smart folks like you, I'm just a lowly biotechnologist and my current job in cancer research doesn't require calculus so I am resigned to being referred to as dumb. Condescend away!!! I don't mind at all.
    Thank you for elucidating that you do not add the suffix together. Then why was everyone in the posts above telling me that if you add two of them together it does not equal one?

    Not trying to be condescending here, but here's by best shot

    one "e to the x" plus one "e to the x" equals 2 "e to the x"

    You add the coefficients and keep e^x the same

    This is not any different from adding
     <br /> <br />
3x^2<br />
to 5x^2

    You have 3 of "something" and you add 5 of "something" so you get 8 "somethings" i.e. 8x^2

    now read what you originally wrote

    e^x+e^x=e^x FALSE
    Follow Math Help Forum on Facebook and Google+

  12. #12
    Banned
    Joined
    Sep 2009
    Posts
    21

    Thank YOU!

    I am so glad that math geniuses like you all exist. It keeps the rest of us from going insane, I tell you, INSANE! Thank you tons, I completely understand now.
    I give you the imaginary medal of "Smarty Pants & Extra Patient Math Helper and Super Explainer of Concepts".
    (sorry, it's late and I've done about 12 hours of calculus today, seriously)
    Follow Math Help Forum on Facebook and Google+

  13. #13
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,367
    Thanks
    1312
    I learned that ac+ bc= (a+ b)c in the seventh grade. And that's all that is being used here: 3e^x+ 5e^x= (3+ 5)e^x= 8e^x. I will concede that I did not start calling that the "distributive law" until much later.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. general question
    Posted in the Geometry Forum
    Replies: 2
    Last Post: October 20th 2010, 07:10 PM
  2. General solution question
    Posted in the Differential Equations Forum
    Replies: 3
    Last Post: July 15th 2010, 04:45 AM
  3. A general question about mean
    Posted in the Statistics Forum
    Replies: 1
    Last Post: December 29th 2009, 06:27 AM
  4. General Question About Sequences
    Posted in the Calculus Forum
    Replies: 2
    Last Post: July 12th 2009, 11:26 AM
  5. A general question..
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: September 5th 2007, 03:16 PM

Search Tags


/mathhelpforum @mathhelpforum