1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Prove the following identity

  1. Jan 15, 2012 #1
    1. The problem statement, all variables and given/known data

    http://img843.imageshack.us/img843/3826/help3c.png [Broken]

    2. Relevant equations

    Not applicable.

    3. The attempt at a solution

    http://img810.imageshack.us/img810/5577/help2n.png [Broken]

    Can anyone prove this by evaluating the left side and right side independent of each other?

    http://img827.imageshack.us/img827/9757/helpdi.png [Broken]

    Can someone explain why this is incorrect? I'm so confused.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 15, 2012 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    [tex] 2^{3^4} = 2^{81} = 2417851639229258349412352, \mbox{ while } 2 \times 3 \times 4 = 24. [/tex] In fact, [tex] ((x^a)^b)^c = x^{a b c},[/tex] as you have recognized, but that is _very_ different from [tex] x^{a^{b^c}},[/tex] which you are confused about. Look at the brackets: in [itex] ((x^a)^b)^c [/itex] we first compute [itex] x^a,[/itex] then take the bth power of that, then take the cth power of the result. In x^[a^(b^c)] we first compute b^c, then raise a to that power, then raise x to the result.

    Last edited by a moderator: May 5, 2017
  4. Jan 15, 2012 #3
    Interesting. Thanks for the clarification.

    If anyone is wondering what I meant by evaluating each side independent of each other, I meant evaluating the left-hand side, and then evaluating the right-hand side, and then after they look exactly the same, you can conclude that the left-hand side equals the right-hand side. Unfortunately, this is the only way my school accepts proofs.
  5. Jan 15, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Operating only on the LHS: you first prove the relation to change base and then use that identity, vis:[tex]LHS=\log_{(a^b)}(c^d) = \frac{\log_a(c^d)}{\log_a(a^b)} = \frac{d\;\log_a(c)}{b}=RHS[/tex]...

    (If you have already done the proof [change-base identity], i.e. in class, you can just refer to it; and if you haven't, you can look it up.)

    Aside: it is probably worth your while learning LaTeX rather than posting images.
    It helps us too since we can then copy and paste from your post to edit your equations.
  6. Jan 15, 2012 #5
    Thanks! That helped a ton! :)

    @LaTeX: I looked at the tutorial, and it looked rather complicated... I've been using Daum Equation Editor (a chrome extension) and it has a beautiful interface. I'll learn LaTeX as soon as this semester is over though.

    http://img860.imageshack.us/img860/1480/32446906.png [Broken]
    Last edited by a moderator: May 5, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook