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jmc

geometry senior

Problem

A cone is created by rotating an isosceles right triangle with leg length 2 about one of its legs. Its surface area is times what number?
Solution
Rotating the triangle about one of its legs produces a cone with radius 2 and height 2:

The base of the cone is a circle with radius 2, which has area .

When unrolled, the curved lateral area of the cone becomes a flat sector of a circle: The sector's radius is the cone's slant height, which, by the Pythagorean theorem, is The sector's arc length is the cone's base perimeter, which is The circle's circumference is so the ratio of the sector's area to the circle's area is . The circle's area is so the sector's area is Summing the lateral area and the base area gives a total surface area of , so its total surface area is times .
Final answer
4\sqrt{2} + 4