Thursday, May 20, 2010

Fox 289

Commonality of the result may imply the existence of a simply solution, which we don't know.
Good luck!


  1. Join the centre of the circle to the top right corner, then n is the shortest and m/n equals the Golden Ratio (1+sqrt5)/2.

  2. golden ratio is indeed the right answer, but not achieved at the point where n is minimal. For an obvious reason: for any parameterized P(m(t),n(t)) representation, when n'=0, m'<>0 and n>0 therefore (m/n)' = m'n-n'm/n^2 = m'/n <> 0. Hence ratio m/n cannot be extremal when n is.

    1st method: O being the center of the circle, take P such as cos(angle(OP,horizontal)) = 2/3.

    2nd method: draw a line from bottom right corner through the point where n is minimal (as constructed previously). This line crosses the circle in a second point which is the requested point.

    Unfortunately, all these results come from differential calculus and trigonometry. Any idea for a geometric justification?


  3. To simplify,

    * The center point of the semicircle is (0,0).
    * The square size is 2 x 2.

    m/n is represented by:

    sqrt((2-cos(a))^2+(-1-sin(a))^2)/sqrt((2-cos(a))^2+(1-sin(a))^2); (-pi/2 < a < pi/2)

    Then I got the max value of m/n as follows.

    Max: (1 + sqrt(5))/2 (= golden ratio) at a = 0.8410..radians

  4. When m/n reaches the max, the coordinates of P is ((2/3,sqrt(5)/3), assuming the center of the semicircle is (0,0).

    m = PB = approx 2.196
    n = PA = approx 1.357
    m/n = approx 1.618 (golden ratio)

    ∠APB = approx 63.45 degrees