Results 1 to 3 of 3

Math Help - Operation on two functions

  1. #1
    Newbie
    Joined
    Jan 2013
    From
    Canada
    Posts
    2

    Operation on two functions

    Hi everyone,

    I know that it may sound like a dumb problem to solve for most of you, but I am a bit rusty and I'm doing my Calculus course remotely. In my first homework, there is a problem where I need to do this:

    f(x)=(√(1-3x))/(x^3)
    g(x)=x^2-x-1


    (fg)(4)

    When I look at this, It seems impossible to me, but Mathway seems to find that the answer is 4.
    I get stuck at the point where everything is now equal to (√(-11))/(64))*(11)
    To me it cannot be solved, am I wrong?

    Thank you to anyone who took the time to read and consider my issue.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Sep 2012
    From
    Australia
    Posts
    3,662
    Thanks
    606

    Re: Operation on two functions

    Hey someone.

    The notation (fg)(x) is such that you work out u = g(x) first and then f(u) after-wards.

    Doing the calculation gives us:

    g(4) = 4^2 - 4 - 1 = 11

    f(11) = SQRT(1 - 3*11)/11^3 = -SQRT(32)/11^3

    Note the definition of composition I'm using can be found here:

    Function composition - Wikipedia, the free encyclopedia.

    Doing it the other way gives us:

    f(4) = SQRT(-11)/64

    g(f(4)) = -11/64^2 + SQRT(-11)/64 - 1

    You may want to bring this up with your tutor.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jan 2013
    From
    Canada
    Posts
    2

    Re: Operation on two functions

    Hi Chiro,

    Thank you for showing me the alternate way you used. The maths you wrote in your post really helped me to understand function operations more easily.

    I'll be sure to discuss that with my tutor once I have the opportunity to do so.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. row operation
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: May 8th 2011, 01:19 PM
  2. commutative operation
    Posted in the Discrete Math Forum
    Replies: 9
    Last Post: July 31st 2010, 03:22 AM
  3. Operation @
    Posted in the Advanced Math Topics Forum
    Replies: 5
    Last Post: April 1st 2010, 11:10 PM
  4. Operation
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: March 24th 2010, 07:20 PM
  5. row operation
    Posted in the Algebra Forum
    Replies: 1
    Last Post: October 17th 2007, 02:39 AM

Search Tags


/mathhelpforum @mathhelpforum