Results 1 to 2 of 2

Math Help - YouTube lesson about precalculus wrong?

  1. #1
    Junior Member
    Joined
    Mar 2009
    Posts
    33

    YouTube lesson about precalculus wrong?



    It starts at 5:29. Basically, what he is saying is that:

    x^{\frac{1}{3}} + x^{\frac{1}{2}} - 7 = 0

    He's says that you can take the LCM of the fractional exponents (6), and multiply both sides of the equation by x^6 to remove the fractions. But, if you do that, wouldn't this happen:

    x^{\frac{6}{1}} * x^{\frac{1}{3}} = x^{\frac{18}{3} + \frac{1}{3}} = x^{\frac{19}{3}}

    Which doesn't help at all.. And even if you tried something like this:

    (x^{\frac{1}{3}} + x^{\frac{1}{2}})^6 = 7^6

    Even using the binomial theorem and expanding, it would probably reintroduce more strange fractional exponents. Am I right?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Banned
    Joined
    Aug 2008
    Posts
    530
    Quote Originally Posted by Phire View Post


    It starts at 5:29. Basically, what he is saying is that:

    x^{\frac{1}{3}} + x^{\frac{1}{2}} - 7 = 0

    He's says that you can take the LCM of the fractional exponents (6), and multiply both sides of the equation by x^6 to remove the fractions. But, if you do that, wouldn't this happen:

    x^{\frac{6}{1}} * x^{\frac{1}{3}} = x^{\frac{18}{3} + \frac{1}{3}} = x^{\frac{19}{3}}

    Which doesn't help at all.. And even if you tried something like this:

    (x^{\frac{1}{3}} + x^{\frac{1}{2}})^6 = 7^6

    Even using the binomial theorem and expanding, it would probably reintroduce more strange fractional exponents. Am I right?
    Yes, you are RIGHT. He is teaching the wrong method.

    Actually it should be

    x^{\frac{1}{3}} + x^{\frac{1}{2}} - 7 = 0

    x^{\frac{2}{6}} + x^{\frac{3}{6}} - 7 = 0

    (x^{\frac{1}{6}})^2 + (x^{\frac{1}{6}})^3 - 7 = 0

    Then, substitute x^{\frac{1}{6}} = t

    It becomes

     t^2+t^3-7=0

     t^3+t^2-7=0

    and solve further.
    Last edited by Shyam; May 8th 2009 at 05:33 PM.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Capturing Handwriting and Spoken Word for YouTube
    Posted in the Math Software Forum
    Replies: 0
    Last Post: December 21st 2011, 02:47 PM
  2. tangents.--missed this lesson
    Posted in the Geometry Forum
    Replies: 2
    Last Post: October 22nd 2008, 08:49 AM
  3. Help with Logic lesson
    Posted in the Discrete Math Forum
    Replies: 0
    Last Post: January 15th 2008, 07:09 AM

Search Tags


/mathhelpforum @mathhelpforum