Results 1 to 6 of 6

Math Help - Find equation of tangent..

  1. #1
    Junior Member
    Joined
    Sep 2010
    Posts
    34

    Find equation of tangent..

    Find the equation of the tangent to the curve f (x) = 2x^2 - 4x + 4 which is perpendicular to the line y = -1/4x + 4.

    I worked it out but got it slightly wrong and I dont know what I did incorrectly.
    Could someone please clarify?

    So what I did was I said okay. The gradient for the tangent is m=4.
    and f'(x)=4x-4 so if the m=4, then x would equal 0.
    If x=0, y=4 when you sub x=0 into f(x).

    so i put it into the formula and got:
    y-4=4(x-0)
    y=4x+4

    The answer is 4x-4.

    Maybe its just a silly mistake that I can't point out and maybe its not. Anyone like to point it out?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Senior Member
    Joined
    Feb 2008
    Posts
    297
    The gradient of the tangent is 4. So when m=4, x=2.


    f'(x)=4x-4\Rightarrow 4=4x-4\Rightarrow x=2

    When x=2, y=4

    Using the point-gradient form

    y-y_1=m(x-x_1)

    y-4=4(x-2)

    y=4x-4
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Sep 2010
    Posts
    34
    Oh i get it now thanks!
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Senior Member
    Joined
    Feb 2008
    Posts
    297
    Since its perpendicular to the line, the product of the gradients of the lines equals -1

    m_{1}m_2=-1\Rightarrow -\frac{1}{4}m_T=-1\Rightarrow m_T=4

    So the gradient of the tangent is 4.

    You know that f'(x)=4x-4, which is the gradient function.

    The gradient equals 4 when f'(x)=4

    4x-4=4\Rightarrow 4x=8\Rightarrow x=2
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Junior Member
    Joined
    Sep 2010
    Posts
    34
    yeah. for some reason before your math equatoins didnt load on your last post thats why i was confused but thanks for the thorough explaination
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Senior Member
    Joined
    Feb 2008
    Posts
    297
    No worries. Have a good one
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Find the equation of the tangent
    Posted in the Calculus Forum
    Replies: 1
    Last Post: January 16th 2012, 10:14 AM
  2. Replies: 8
    Last Post: March 22nd 2011, 04:57 PM
  3. Replies: 5
    Last Post: May 24th 2009, 12:15 PM
  4. Find the equation of the tangent
    Posted in the Calculus Forum
    Replies: 0
    Last Post: November 24th 2008, 11:21 AM
  5. Find an equation of tangent
    Posted in the Calculus Forum
    Replies: 4
    Last Post: May 7th 2008, 04:33 AM

/mathhelpforum @mathhelpforum