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: Related Rates Problems

  1. Sep 24, 2007 #1
    Let x & y be differentiable functions of t and let s = sqrt(x^2+y^2) be the distance between the points (x,0) and (0,y) in the xy-plane.

    How is ds/dt related to dx/dt if y is constant?

    So I attempted to implicitly take the derivatives of the changing rates.

    ds/dt= 1/(2sqrt(x^2+y^2)) times 2x dx/dt + 2y

    Which simplifies to

    ds/dt= x/(sqrt(x^2+y^2)) dx/dt + y/(sqrt(x^2+y^2))

    I'm guessing there is something I'm not understanding because he book shows an answer of ds/dt= x/(sqrt(x^2+y^2)) dx/dt

    So what y does the y disappear? Or what am I doing incorrectly?
  2. jcsd
  3. Sep 25, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Ummm. "if y is constant" says the problem. On such a path, dy/dt=0.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook