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smc

geometry senior

Problem

Two equal parallel chords are drawn inches apart in a circle of radius inches. The area of that part of the circle that lies between the chords is:
(A)
(B)
(C)
(D)
Solution
Draw in the diameter through A perpendicular to chords BD and CE. Label the intersection points of the diameter with the chords F and G. Then, AF=4, AG=4, and AFB ,AFD, AEG. and AGC are right triangles. We're going to find the area inside sectors BAD and CAE but outside triangles BAD and CAE. By pythagorean theorem, BF=, as are DF, EG, and GC. It then follows that the area of triangles BAD and CAE are . Since AFB and AFD are 30-60-90 triangles, the area of sector BAD is , as is sector CAE. Thus, the area outside of the two chords is . Since we want the area inside the two chords, you can subtract the outside from the whole circle, which is , or .
Final answer
B