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How do you convert from one line equation to another?

  1. Oct 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Show how the equation of a line y-2=4x-4 can be converted into the equivalant line equation of 4x+y-6=0.

    2. Relevant equations



    3. The attempt at a solution

    The closest I have got to it is 4x-y-2=0 by adding and subtracting terms to both sides.
     
  2. jcsd
  3. Oct 7, 2010 #2

    Mark44

    Staff: Mentor

    It can't. These two equations aren't equivalent.

    y - 2 = 4x - 4
    <==> y - 4x + 2 = 0
    I got this by adding -4x and +4 to both sides.
    This equation is equivalent to the one you show below. Multiply the equation above by -1 on both sides and you'll get an equation that can be rearranged to yours.
     
  4. Oct 7, 2010 #3
    Thanks for the help. The reason I asked this question is to find the equation of the tangent to the circle (x+3)^2 + (y-1)^2=17 at the point (1,2). I know the answer to this is 4x+y-6=0. Can you explain to me how to get this answer.
     
  5. Oct 7, 2010 #4

    Mark44

    Staff: Mentor

    That answer is correct. I think you made a mistake in calculating the derivative. The slope of the tangent line at (1, 2) is -4, not 4, as you show.
     
  6. Oct 7, 2010 #5
    Yes sorry, what I asked in the original question was wrong. Can you explain how to get the answer 4x+y-6=0 for the equation of the tangent from the circle equation and the point I gave in my previous reply. Thanks.
     
  7. Oct 7, 2010 #6

    Mark44

    Staff: Mentor

    1. Find dy/dx using the circle equation. I used implicit differentiation.
    2. Evaluate dy/dx at (1, 2) to get the slope of the tangent line.
    3. Use the slope found in step 2 and the point (1, 2) in the point slope form of the line.
     
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