2015 AIME I Problems/Problem 15
A block of wood has the shape of a right circular cylinder with radius and height , and its entire surface has been painted blue. Points and are chosen on the edge of one of the circular faces of the cylinder so that on that face measures . The block is then sliced in half along the plane that passes through point , point , and the center of the cylinder, revealing a flat, unpainted face on each half. The area of one of these unpainted faces is , where , , and are integers and is not divisible by the square of any prime. Find .
Label the points where the plane intersects the top face of the cylinder as and , and the center of the cylinder as , such that and are collinear. Let be the center of the bottom face, and the midpoint of . Then , (because of the 120 degree angle), and so .
Project and onto the bottom face to get and , respectively. Then the section (whose area we need to find), is a stretching of the section on the bottom face. The ratio of stretching is , and we do not square this value when finding the area because it is only stretching in one direction. Using 30-60-90 triangles and circular sectors, we find that the area of the section is . Thus, the area of section is , and so our answer is .
Label the points same as in the first sentence above. Consider a view of the cylinder such that height is disregarded, i.e. a top view. From this view, note that Cylinder has become a circle with = = . Using 30-60-90 triangles, we get rectangle to have a horizontal component of . Now, consider a side view, such that and coincide at the bottom of the diagram. From this view, consider the right triangle composed of hypotenuse and a point along the base of the viewpoint, which will be labeled as . From the top view, . Because of the height of the cylinder, is equal to . This makes equal to .
Now, the use of simple calculus is required. Conceptualize an infinite number of lines perpendicular to intersecting both and . Consider the area between point and the first vertical line. Label the point where the line intersects AE as E', and the point where the line intersects AD as D'. The area of the part of the initial unpainted face within these two positions approaches a rectangle with length AD' and width . The area of the base within these two positions approaches a rectangle with length AE' and width . The ratio of AD':AE' is 10:6, since the ratio of AD:AE is 10:6. This means that the area of the initial unpainted surface within these two positions to the area of the base within these two positions is equal to 10:6 = 10:6. Through a similar argument, the areas between each set of vertical lines also maintains a ratio of 10:6. Therefore, the ratio of the area we wish to find to the area of the base between AB and CD (from the top perspective) is 10:6. Using 30-60-90 triangles and partial circles, the area of the base between AB and CD is calculated to be . The area of the unpainted surface therefore becomes , and so our answer is .
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