Results 1 to 9 of 9

Math Help - Calculas!

  1. #1
    Member Awsom Guy's Avatar
    Joined
    Jun 2009
    From
    Mars
    Posts
    186

    Calculas!

    I have another question to post here.
    Write the equation of the straight line passing through the point (2,1) and normal to the curve 4y=x^2.

    I know one thing x=2 and y=1 but I have no idea where to start or to do anything. How does the equation shown derivate.
    Please help me get started.

    The answer provided is x+y-3=0.
    Thanks
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Prove It's Avatar
    Joined
    Aug 2008
    Posts
    11,506
    Thanks
    1401
    Quote Originally Posted by Awsom Guy View Post
    I have another question to post here.
    Write the equation of the straight line passing through the point (2,1) and normal to the curve 4y=x^2.

    I know one thing x=2 and y=1 but I have no idea where to start or to do anything. How does the equation shown derivate.
    Please help me get started.

    The answer provided is x+y-3=0.
    Thanks
    The curve is 4y = x^2

    or y = \frac{1}{4}x^2.


    It is easy to see that (2, 1) lies on this curve.


    \frac{dy}{dx} = \frac{1}{2}x

    and when x = 2, \frac{dy}{dx} = 1.


    So the tangent will have gradient = 1, which means the normal will have gradient -\frac{1}{1} = -1.


    So the normal has gradient -1 and passes through (2, 1)

    y = mx + c

    1 = -1(2) + c

    1 = -2 + c

    c = 3.


    So the equation of the normal is y = -x + 3.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member Awsom Guy's Avatar
    Joined
    Jun 2009
    From
    Mars
    Posts
    186

    sorry one more little thing...

    one little question is the derivative of 2 equal to 1.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    Quote Originally Posted by Awsom Guy View Post
    one little question is the derivative of 2 equal to 1.

    no, \frac{d}{dx}(2)= 0
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Member Awsom Guy's Avatar
    Joined
    Jun 2009
    From
    Mars
    Posts
    186
    Then why is the above derivative of 2 = 1. explain please.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    MHF Contributor
    Prove It's Avatar
    Joined
    Aug 2008
    Posts
    11,506
    Thanks
    1401
    The derivative is \frac{1}{2}x.

    What does this equal when x = 2?
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Master Of Puppets
    pickslides's Avatar
    Joined
    Sep 2008
    From
    Melbourne
    Posts
    5,236
    Thanks
    28
    I think you are confsued, Prove it has said if

    y = \frac{1}{4}x^2 then \frac{dy}{dx} = \frac{1}{2}x

    This is the only derivate that has been taken.

    The following is not a derivative, Prove it is saying at the point x = 2, then \frac{dy}{dx} = 1

    I.e from the first statement

    \frac{dy}{dx} = \frac{1}{2}x

    Put x = 2 here gives \frac{dy}{dx} = \frac{1}{2}\times 2 =1
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Member Awsom Guy's Avatar
    Joined
    Jun 2009
    From
    Mars
    Posts
    186
    Right thanks for explaining that. I appreciate that.
    Follow Math Help Forum on Facebook and Google+

  9. #9
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,569
    Thanks
    1410
    The derivative of \frac{1}{4}x^2, \frac{1}{2}x at x= 2 is 1. The "derivative of 2" means the derivative of the constant function "f(x)= 2" which is 0 because the derivative measures the rate of change and a constant function doesn't change.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Calculas question
    Posted in the Pre-Calculus Forum
    Replies: 5
    Last Post: February 8th 2010, 12:59 AM
  2. need help with calculas questions
    Posted in the Calculus Forum
    Replies: 0
    Last Post: September 30th 2009, 07:44 AM
  3. Calculas - Differentiability
    Posted in the Calculus Forum
    Replies: 1
    Last Post: March 21st 2009, 05:52 AM
  4. Help Calculas
    Posted in the Calculus Forum
    Replies: 19
    Last Post: October 24th 2008, 07:32 PM
  5. integral calculas???
    Posted in the Calculus Forum
    Replies: 3
    Last Post: November 1st 2007, 04:10 AM

Search Tags


/mathhelpforum @mathhelpforum