Results 1 to 4 of 4

Thread: Simple as it sounds?

  1. #1
    Newbie
    Joined
    May 2018
    From
    toronto
    Posts
    1

    Simple as it sounds?

    Cousin had this question:

    Determine the point (a,f(a)) for which f'(a)=a given that f(x)=-x^2+3x-7

    We both did this question the same way, got the same solution, but textbook disagrees.

    Can anyone try so we know we're not going crazy?

    Thanks
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    11,207
    Thanks
    755
    Awards
    1

    Re: Simple as it sounds?

    Quote Originally Posted by boyohboy View Post
    Cousin had this question:

    Determine the point (a,f(a)) for which f'(a)=a given that f(x)=-x^2+3x-7

    We both did this question the same way, got the same solution, but textbook disagrees.

    Can anyone try so we know we're not going crazy?

    Thanks
    We can't catch a possible mistake unless we know what you did. So... How did you solve this?

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Joined
    Nov 2013
    From
    California
    Posts
    6,180
    Thanks
    2632

    Re: Simple as it sounds?

    $f(x) = -x^2 + 3x -7$

    $f'(x) = -2x + 3$

    solve

    $a = -2a+3$

    $3a = 3$

    $a=1$

    are you going crazy?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor

    Joined
    Aug 2006
    Posts
    21,878
    Thanks
    2887
    Awards
    1

    Re: Simple as it sounds?

    Quote Originally Posted by boyohboy View Post
    Determine the point (a,f(a)) for which f'(a)=a given that f(x)=-x^2+3x-7
    We both did this question the same way, got the same solution, but textbook disagrees.
    $ \begin{align*}f(x)&=-x^2+3x-7 \\f^{\prime}(x)&=-2x+3\\\\a&=-2a+3\\3a&=3\\a&=1 \end{align*}$
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. sounds like old tracks from
    Posted in the Geometry Forum
    Replies: 0
    Last Post: Apr 17th 2015, 04:39 AM
  2. Bats emit sounds...
    Posted in the Advanced Applied Math Forum
    Replies: 5
    Last Post: Aug 7th 2006, 06:20 PM

/mathhelpforum @mathhelpforum