Problem 8 · 2015 Math Kangaroo
Medium
Spatial & Visual Reasoning
net-folding
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Answer: E
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Hint 1 of 2
Imagine slitting the glass along one slant line and rolling it flat.
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Hint 2 of 2
The top and bottom rims become circular arcs of different radii.
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Approach: unroll the lateral surface of the frustum
- Cutting the side of the truncated cone along a slant line and flattening it gives a piece of a sector centred at the apex of the full cone (the cone you get by extending the glass to a point).
- The top and bottom rims flatten into two concentric circular arcs (the wider bottom rim becomes the longer outer arc), joined by two straight slant edges — an annular sector.
- That is the shape with two arcs and the apex shown by dashed lines (E).
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