A unit circle is divided into halves. One of these pieces is divided into two halves, and one of those pieces is divided into two halves, and so on. Each of these sectors of the unit circle is rolled up into a cone-shape, and a circular base is capped onto each of the cones. The total surface area of all of these cones combined can be expressed as for positive coprime integers . What is ?
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First, note that the sum of all the lateral faces of the cones is the same as the area of the unit circle, which is π .
Now all we need to do is find the sum of the areas of the circular bases.
We can see that the circumference of the circular bases are 2 1 th, 4 1 th, 8 1 th etc. of the circumference of the unit circle.
Since the circumference of the unit circle is 2 π , the circumferences of the bases are π , 2 π . 4 π , … .
If the circumference of the base is n π , then the radius is 2 π n π = 2 n , and the area is 4 n 2 π .
Therefore, the sum of the areas of all the bases is 4 π + 1 6 π + 6 4 π + ⋯ = 1 − 4 1 4 π = 3 π .
Our final answer, is therefore π + 3 π = 3 4 π , and our answer is 4 + 3 = 7 .