1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Tangents to the curves

  1. Oct 1, 2012 #1
    1. The problem statement, all variables and given/known data

    Is there anything special about the tangents to the curves xy=1 and (x^2) - (y^2) =1 at their point of intersection in the first quadrant.

    3. The attempt at a solution

    I know what the derivatives of both functions are and what they look like when graphed. But, I'm not sure what to do with it beyond that. For the first function I have that dy/dx=-1/x^2 and for the second I have dy/dx = x/sqrt(-1+x^2). What is different about these functions and what happens in the first quadrant?
  2. jcsd
  3. Oct 1, 2012 #2


    Staff: Mentor

    Re: URGENT: tangents to the curves

    Well, according to the problem statement, the functions intersect there. Seems to me that it would be useful to find the point of intersection.
  4. Oct 1, 2012 #3
    Re: URGENT: tangents to the curves

    I have been trying to do that. However, I have not been able to find the intersection of the tangents and there is not intersection of the original functions.
  5. Oct 1, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: URGENT: tangents to the curves

    There are in fact two points of intersection. Why do you think there are none?

    You need to find (x,y) that satisfies both xy = 1 and x^2 - y^2 = 1. How do you solve two equations with two unknowns?
  6. Oct 1, 2012 #5


    User Avatar
    Science Advisor

    Re: URGENT: tangents to the curves

    You don't need to find where they intersect.

    With xy= 1, xy'+ y= 0 so y'= -y/x. With [itex]x^2- y^2= 1[/itex], [itex]2x- 2yy'= 0[/itex] so y'= x/y. Without worrying about what the "points of intersection" are, what is the product of thos two derivatives for a given (x, y)?
  7. Oct 1, 2012 #6
    Re: URGENT: tangents to the curves

    Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook