jmc

Question

Jenny and Kenny are walking in the same direction, Kenny at 3 feet per second and Jenny at 1 foot per second, on parallel paths that are 200 feet apart. A tall circular building 100 feet in diameter is centered midway between the paths. At the instant when the building first blocks the line of sight between Jenny and Kenny, they are 200 feet apart. Let t, be the amount of time, in seconds, before Jenny and Kenny can see each other again. If t, is written as a fraction in lowest terms, what is the sum of the numerator and denominator?

Accepted Answer

Consider the unit cicle of radius 50. Assume that they start at points (-50,100) and (-50,-100). Then at time t, they end up at points (-50+t,100) and (-50+3t,-100). The equation of the line connecting these points and the equation of the circle arealigny&=-100tx+200-5000t50^2&=x^2+y^2align.When they see each other again, the line connecting the two points will be tangent to the circle at the point (x,y). Since the radius is perpendicular to the tangent we get-xy=-100tor xt=100y. Now substitutey= xt100into (2) and getx=5000100^2+t^2.Now substitute this andy=xt100into (1) and solve for t to gett=1603.Finally, the sum of the numerator and denominator is 160+3=163.