Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1993 based on Equation of X and Y.
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 centred mid way 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 amount of time, in seconds, Before Jenny and Kenny, can see each other again. If t is written as a fraction in lowest terms, find the sum of numerator and denominator.
Variables
Equations
Algebra
Answer: is 163.
AIME I, 1993, Question 13
Elementary Algebra by Hall and Knight
Let circle be of radius 50
Let start points be (-50,100),(-50,-100) then at time t, end points (-50+t,100),(-50+3t,-100)
or, equation and equation of circle is
y=\(\frac{-100}{t}x+200 -\frac{5000}{t}\) is first equation
\(50^2=x^2+y^2\) is second equation
when they see again then
\(\frac{-x}{y}=\frac{-100}{t}\)
or, \(y=\frac{xt}{100}\)
solving in second equation gives \(x=\frac{5000}{\sqrt{100^2+t^2}}\)
or, \(y=\frac{xt}{100}\)
solving in first equation for t gives \(t=\frac{160}{3}\)
or, 160+3=163.
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