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: Circle Question

  1. Oct 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Show that the point (3/5, 4/5) is the closest point on the circle x[tex]^{2}[/tex] + y[tex]^{2}[/tex] = 1 to the point (3, 4).

    2. Relevant equations


    3. The attempt at a solution
    The only equation that comes to mind is the distance formula... but I don't know what I would show with that. I know the solution deals with calculus, I just don't know what concepts to apply.
  2. jcsd
  3. Oct 18, 2009 #2
    Why don't you write down the distance formula for any point on the circle to the point (3,4), and show that the minimum occurs at the point (3/5, 4/5)?
  4. Oct 18, 2009 #3
    Ah, that makes sense. One question, though... do I have to solve the original equation for x1 and y1 before substituting into the distance equation? (I just don't see how you set up the circle equation and various points via the distance equation.) From there, though, I know how to find the minimum.
  5. Oct 18, 2009 #4


    User Avatar
    Science Advisor

    You don't have to solve for y- you can use "inplicit differentiation". But you don't have to differentiate at all:

    One crucial point about "min distance" is that the line from a point to a curve that is of minimum distance is perpendicular to the (tangent to the) curve. And, since this is a circle, a tangent is always perpendicular to the radius. The minimum distance from a point to a circle is always measured along the extended radius of the circle.

    Since a radius goes through the center, what is the equation of the line through the center of this circle and (3,4)? where does that line intersect the circle? (There will be two points where that line intersects the circle, of course, but one is obviously "nearest", the other "farthest" from the point.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook