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Geometry Difficulty 3.6 AMC 10/12 Find the answer

Semicircle AB^\widehat{AB} has center CC and radius 11. Point DD is on AB^\widehat{AB} and CDAB\overline{CD}\perp\overline{AB}. Extend BD\overline{BD} and AD\overline{AD} to EE and FF, respectively, so that circular arcs AE^\widehat{AE} and BF^\widehat{BF} have BB and AA as their respective centers. Circular arc EF^\widehat{EF} has center DD. The area of the shaded "smile" AEFBDAAEFBDA, is

Pick one

Solution

B\fbox{B} The area of the entire outer shape is the area of sector ABEABE, plus the area of sector ABFABF, minus the area of triangle ABDABD (since it is part of both sectors), plus the area of sector DEFDEF. We know AC=CD=1AC = CD = 1, so the sector angles for ABEABE and ABFABF are 4545 degrees, and the radius of both of them is 22. The radius of DEFDEF is DE=BEBD=2BDDE = BE - BD = 2 - BD, and BDBD can be found using Pythagoras in triangle BCDBCD, giving BD=2BD = \sqrt{2} and DE=22DE = 2 - \sqrt{2}, so after doing all the calculations, the area of the entire outer shape is π(522)1\pi(\frac{5}{2} - \sqrt{2}) - 1. To get the area of the smile, we need to subtract the area of semicircle ABDABD, which is 12π12=π2\frac{1}{2} \pi 1^2 = \frac{\pi}{2}, so the answer is π(52122)1\pi(\frac{5}{2} - \frac{1}{2} - \sqrt{2}) - 1 = 2ππ212\pi - \pi \sqrt{2} - 1.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.