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: Common tangents equations

  1. Aug 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Finding the equations of common tangents of [itex]y^{2}[/itex]=4ax and [itex]x^{2}[/itex]=4by

    2. Relevant equations

    Equating both curves' derivatives :

    3. The attempt at a solution

    Calculated both of the curves' derivatives and equated them .
    Used the resulting relation in two point form of straight line equation (y-y1)=m(x-x1) .

    Got Stuck now! with too many cuberoots .
  2. jcsd
  3. Aug 6, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    In your equation: [itex]\displaystyle \frac{2a}{y}=\frac{x}{2b}\,,[/itex] the y is from a point on the first curve, the x from a point on the second curve.

    Instead, start by doing the following:

    Find the general equation for a line tangent to y2=42x.

    The find the general equation for a line tangent to x2=4by.
    Last edited: Aug 6, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook