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imc

geometry intermediate

Problem

Four congruent semicircles are drawn on the surface of a sphere with radius , as shown, creating a close curve that divides the surface into two congruent regions. The length of the curve is . What is ?
(A)
(B)
(C)
(D)
Solution
There are four marked points on the diagram; let us examine the top two points and call them and . Similarly, let the bottom two dots be and , as shown: This is a cross-section of the sphere seen from the side. We know that , and by Pythagorean Theorem, length of Each of the four congruent semicircles has the length as a diameter (since is congruent to and ), so its radius is Each one's arc length is thus We have of these, so the total length is , so thus our answer is Note: TLDR: The radius of gives us a line segment connecting diagonal vertices of the semi-circles with a measure of , giving us through relations and Pythagorean theorem a diameter for each semi-circle of , which we can use to bash out the circumference of a full circle, multiply by , and move inside and under the root to get .
Final answer
A