Page 2 of 2 FirstFirst 12
Results 16 to 21 of 21

Math Help - Proving a function is onto.

  1. #16
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,605
    Thanks
    1574
    Awards
    1

    Re: Proving a function is onto.

    Quote Originally Posted by mathproblems View Post
    yes, this is what I am saying. I went to school 25 years ago...now I don't remember some things. And now I am taking discrete math ...
    Please go back and sit in of a basic algebra class.
    If you do not, you are going to be frustrated beyond belief by the material you will meet in a discrete mathematics course.
    Follow Math Help Forum on Facebook and Google+

  2. #17
    Junior Member
    Joined
    Oct 2011
    Posts
    47

    Re: Proving a function is onto.

    too late, I am already in the middle of semester.
    Follow Math Help Forum on Facebook and Google+

  3. #18
    Super Member TheChaz's Avatar
    Joined
    Nov 2010
    From
    Northwest Arkansas
    Posts
    600
    Thanks
    2

    Re: Proving a function is onto.

    Quote Originally Posted by mathproblems View Post
    too late, I am already in the middle of semester.
    Then maybe you would benefit from some 1-1 help?

    (pun intended)
    Follow Math Help Forum on Facebook and Google+

  4. #19
    Junior Member
    Joined
    Oct 2011
    Posts
    47

    Re: Proving a function is onto.

    yes, I got in a person tutor. I still try to do my homework by myself first.

    anyway, could someone please explain the f (n) =?
    Follow Math Help Forum on Facebook and Google+

  5. #20
    MHF Contributor

    Joined
    Mar 2011
    From
    Tejas
    Posts
    3,319
    Thanks
    697

    Re: Proving a function is onto.

    we need to find an x so that f(x) = c, for any c we choose. so x should be "some expression (formula) in c".

    what is f(x)? by the definition:

    f(x) = 2x^3 - 4. so if f(x) = c, we have:

    2x^3 - 4 = c. now, we need to try to "solve for x".

    2x^3 = c+4 (adding 4 to both sides)

    x^3 = \frac{c+4}{2} (dividing both sides by 2)

    x = \sqrt[3]{\frac{c+4}{2}} (taking the cube root of both sides). this is the x we are looking for (we hope).

    to check, we verify that f(x) is indeed c, when x = \sqrt[3]{\frac{c+4}{2}}.

    f\left(\sqrt[3]{\frac{c+4}{2}}\right) = 2\left(\sqrt[3]{\frac{c+4}{2}}\right)^3 - 4

    = 2\left(\frac{c+4}{2}\right) - 4 = (c+4) - 4 = c + (4-4) = c+0 = c,

    so we indeed found an x that f sends to c.
    Follow Math Help Forum on Facebook and Google+

  6. #21
    Junior Member
    Joined
    Oct 2011
    Posts
    47

    Re: Proving a function is onto.

    oh wow!! Thank you!!! Now I got it!
    Follow Math Help Forum on Facebook and Google+

Page 2 of 2 FirstFirst 12

Similar Math Help Forum Discussions

  1. Proving an onto function
    Posted in the Discrete Math Forum
    Replies: 4
    Last Post: May 1st 2011, 03:02 PM
  2. Proving if a function is odd or even
    Posted in the Pre-Calculus Forum
    Replies: 4
    Last Post: January 6th 2011, 07:47 PM
  3. proving the function is even
    Posted in the Calculus Forum
    Replies: 2
    Last Post: April 17th 2010, 05:05 AM
  4. Proving a function is one-to-one/onto
    Posted in the Discrete Math Forum
    Replies: 2
    Last Post: October 5th 2009, 04:20 PM
  5. Proving a function is one-to-one
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: November 26th 2007, 08:14 PM

Search Tags


/mathhelpforum @mathhelpforum