Search Loci: Convergence:
I recognize the lion
by his paw.
In G. Simmons, Calculus Gems, New York: McGraw Hill, 1992, p. 136.
Thomas Simpson and Maxima and Minima
Motion of bodies I
Two bodies move at the same time, from two given places A and B, and proceed uniformly from thence in given directions, AP and BQ, with celerities in a given ratio; it is proposed to find their position, and how far each has gone, when they are nearest possible to each other (Example XIII, page 28).
Let a = AC, b = BC, c = DC in the adjacent figure. Assume m is the velocity ("celerity") of the body that moves in the direction AP while n is the velocity of the body that moves in the direction BN.
At time t the first body will be at M while the second body will be at N (Simpson calls these two points "cotemporary"). Next draw perpendiculars NE and BD to AP, and let x = CN. Since DECN is similar to DDCB, we can conclude that b/x = c/CE. Because M and N are cotemporary points it follows that AM/m = BN/n, therefore AM = m/n(x - b). So CM = AC - AM = a - m/n(x - b) = d - m/nx, where d = a + m/nb. By the law of cosines we have MN2 = CM2 + CN2 - 2(CM)(CN)cosC. But cosC = EC/CN. Therefore
Taking the derivative of MN2, which is obviously a function of x, and making it equal to zero we get
Remark: Maybe Simpson should have mentioned that MN2 is a second degree polynomial in the variable x, thus on the Cartesian plane it represents a vertical parabola that opens upwards with minimum at -r/2s where r and s are the coefficients of x and x2 respectively. We would get exactly the same value obtained before, namely
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