Embarrassing Precal Question

Discussion in 'Community Discussion' started by poppe, Apr 18, 2007.

  1. poppe macrumors 68020

    poppe

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    #1
    Hello,

    I'm helping my girlfriend with a precal assignment. It is such an embarrassing problem since the problem is quite simple. I am curious if you all could help me. The book she is using is horrendous and does not explain anything that she needs to know (I've already taken calculus and crap).

    Equation:

    5 ^ (1-2x) = 1/5th

    ^ means "to the power of"

    If you could just help me refresh my memory since it's been a little over a year now since I have taken any hard core math...
     
  2. swiftaw macrumors 603

    swiftaw

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    #2
    I presume you are trying to solve for x.

    Well 1/5th is 5^(-1) thus (1-2x) = -1 thus x=1


     
  3. EricNau Moderator emeritus

    EricNau

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    #3
    I believe the answer is (highlight for answer): x=1

    If you multiply natural log by both sides, you can move the exponent to the front, like this:

    (1-2x) log5 = log(1/5)​

    Then you divide log5 on both sides, getting:

    1-2x = [log(1/5)/log5]​

    ...Then you use a calculator to solve the logs, which gives you:

    1-2x = -1​

    Then you finish with basic algebra.


    EDIT: I always was slow at math. :eek:
     
  4. swiftaw macrumors 603

    swiftaw

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    #4
    Or, remember that log(1/5) = log 1 - log 5 = 0 - log 5 = -log 5

    Thus, 1-2x = -log 5 / log 5 = -1

    And thus x = 1
     
  5. poppe thread starter macrumors 68020

    poppe

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    #5
     
  6. swiftaw macrumors 603

    swiftaw

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    #6
    Neither of my answers required a calculator
     
  7. iAlan macrumors 65816

    iAlan

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    #7
    The answer is 1







    But I cheated and looked at the other members answers!
    I can't even get my avatar right!

    :p
     
  8. EricNau Moderator emeritus

    EricNau

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    #8
    You're right, I had forgotten about that!

    If I had known that calculators weren't allowed I probably would've looked a little harder for an easy way to solve it.

    Thanks.
     
  9. poppe thread starter macrumors 68020

    poppe

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    #9
    Sorry I wasn't quoting your. The poster above your quote said use your calculator and I was just explaining that she can't use her calculator.

    Thanks guys... I think I'm gonna have to whip out my old Trig book and see if I can help her better (that is if the assignment isn't due soon)
     
  10. swiftaw macrumors 603

    swiftaw

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    #10
    No problemo
     
  11. siurpeeman macrumors 603

    siurpeeman

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    #11
    i love answering these questions, but i usually go on macrumors late at night when these questions have already been answered. i know, i'm lame. :(


    you could write out the equation on grapher, take a snapshot and post the image. an example would look like the following:
     

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  12. poppe thread starter macrumors 68020

    poppe

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    #12
     
  13. siurpeeman macrumors 603

    siurpeeman

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    #13
    i'm sure if i don't, someone will. :p
     
  14. poppe thread starter macrumors 68020

    poppe

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    #14
    Sounds good.
     

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  15. Eraserhead macrumors G4

    Eraserhead

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    #15
    3^(x+1)=81

    As 81=3^4

    so by powers as before this says that:
    x+1=4

    So therefore.

    x=3

    get her another decent book on the subject (all Maths is the same so there's no harm), look at the reviews on Amazon, they should help you find if they are any good...

    LATE EDIT: Basically with these problems try and turn one number on the bottom into a power of the other, then you can use (relatively) simple maths to find the answer.
     
  16. poppe thread starter macrumors 68020

    poppe

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    #16
    Sorry for Double Post

    More
     

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  17. poppe thread starter macrumors 68020

    poppe

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    #17
    I'm giving her my Trig book when I get back to my house. It was everything she's done in PreCal so far but with a million more explanations and demonstrations.
     
  18. siurpeeman macrumors 603

    siurpeeman

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    #18
    four has a base of two, so you want to change the base to reflect that. once the bases are the same (2), you set the exponents equal. then you just solve.
     

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  19. poppe thread starter macrumors 68020

    poppe

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    #19
    Last Post

    Whats worse than the book itself and it being so crappy is that her teacher, when she (along with other students) ask him a question the teacher would respond: If you don't know you probably shouldn't be in this class.
     

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  20. siurpeeman macrumors 603

    siurpeeman

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    #20
    all these problems are pretty much the same. all the numbers that you see have a common base. for example, numbers like 2, 4, 8, 16, 32 all have a base of 2. 3, 9, 81, and such have a base of 3. take these numbers and reduce to the common base. it's the best way to solve.
     

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  21. poppe thread starter macrumors 68020

    poppe

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    #21
    Oh man.... Now this is all coming back... I can't believe I forgot these problems of all problems... Thanks a lot for all this help. This is helping so much!!!
     
  22. siurpeeman macrumors 603

    siurpeeman

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    #22
    now i'm not going to do all of them, as they're all pretty much variations of the same problem. but i think you can get the general idea of what to do from here on out. if your gf is a college student, i imagine her school would offer free tutuoring on campus to help her with this sort of thing. maybe you/she could look into it. good luck!

    btw, it's almost 4:30 in the morning. i take no responsibility for any mistakes in my work. :)
     

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  23. furious macrumors 65816

    furious

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    #23
    Why are you not my Teacher. I do quants and we did differentiation. Boy is it hard. Having only do general math is high school. :eek: You can guess how much I suck at math.
     
  24. poppe thread starter macrumors 68020

    poppe

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    #24

    Hey thanks so much! She is getting tutoring (or so she tells me) She let her 2 Biology classes and her 2 Biology Labs take over in priority so Math came last and well now... she has screwed herself for tomorrow's homework, well that was until you guys all helped. Thank you all so much!!!!


    EDIT: Just to let you all know. I have worked the remainder and have gotten them all done. thank you so much again.
     
  25. MongoTheGeek macrumors 68040

    MongoTheGeek

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    #25
    When in doubt the answer is 0,1,-1 or if it involves circles pi.
     

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