Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Prove it Bulgaria

Given the functions f(x)=x2x4f(x) = |x-2| - |x-4| and g(x)=x82g(x) = |x-8| - 2. Find the area of the figure with vertices, the intersection points of the graphs of the functions f(x)f(x) and g(x)g(x) and the intersection points of the graph of g(x)g(x) with the x-axis.
(Nedyalka Dimitrova)

Solution

The graph of g(x)g(x) consists of two rays with a common vertex at x=8x = 8. We remove the module and easily calculate its intersection points with the x-axis through the equations 6x=06 - x = 0 and x10=0x - 10 = 0A(6,0)A(6, 0) and B(10,0)B(10, 0).

After removing the modules in f(x)f(x) we see that in the interval (,2)(-\infty, 2) f(x)=2f(x) = -2, in the interval [2,4][2, 4] f(x)=2x6f(x) = 2x - 6 and in the interval (4,)(4, \infty) f(x)=2f(x) = 2. We solve the equations f(x)=g(x)f(x) = g(x) for each of the three intervals. We get the following intersection points — D(4,2)D(4, 2) and C(12,2)C(12, 2).

It follows that the figure ABCDABCD is a trapezoid with bases 88 and 44 and height 22. Therefore, its area is 8+42×2=12\frac{8+4}{2} \times 2 = 12.

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