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Visualizing the Curved Surface Area of Cones

Visualizing the Curved Surface Area of Cones
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2019-05-12 20:02
CircleTrigonometrySolid GeometryGeometric Construction
This example demonstrates how the curved surface of a cone can be flattened into a sector, which is then dissected and rearranged to show that its area is πrl, where r is the radius of the base and l is the slant height. Key math concepts include cones, surface area, sectors, and pi. Construction highlights the use of GeoGebra commands and objects like Point, If, Sequence, CircleArc, Segment, Midpoint, Circle, Intersect, Rotate, Surface, CircumcircleArc, Vector, and Angle. Suitable for high school students learning solid geometry.

GeoGebra Commands & Parameters

  1. Point(CircleArc[(0, 1), (1, 1), (0, 0)]) → Q
  2. If(y(Q) > 0, CircleArc[(0, 1), (0, 0), Q], 6.283185307179586) → s
  3. If(y(Q) > 0, Angle[(0, 0), (0, 1), Q], 6.283185307179586) → α'
  4. Sequence((sin((i * θ)), 1 - cos((i * θ))), i, 0, (2 * n)) → list1
  5. If(t ≤ 1, t, 1) → k
  6. If(t ≥ 1, t - 1, 0) → k'
  7. Sequence(If[k < 1, (x(O_1) + ((l * sin((i * φ))) * cos((i * φ)) / sin(φ)), y(O_1) + ((l * sin((i * φ))) * sin((i * φ)) / sin(φ))), (x(O_1) + (l * i), y(O_1))], i, 0, (2 * n)) → list2
  8. Sequence(Sector[Rotate[Translate[Circle[(x(O_1) + 0, y(O_1) + 1), Element[list1, i]], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]], Rotate[Translate[Element[list1, i], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]], Rotate[Translate[Element[list1, i + 1], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]]], i, 1, n) → list3
  9. Sequence(Sector[Rotate[Rotate[Translate[Circle[(x(O_1) + 0, y(O_1) + 1), Element[list1, i]], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]], (180° * k'), (x(O_1) + (n * l) - sin(θ / 2) / 2, y(O_1) + cos(θ / 2) / 2)], Rotate[Rotate[Translate[Element[list1, i], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]], (180° * k'), (x(O_1) + (n * l) - sin(θ / 2) / 2, y(O_1) + cos(θ / 2) / 2)], Rotate[Rotate[Translate[Element[list1, i + 1], Vector[Element[list1, i], Element[list2, i]]], (((-((2 * i) - 1)) * k) * θ / 2), Element[list2, i]], (180° * k'), (x(O_1) + (n * l) - sin(θ / 2) / 2, y(O_1) + cos(θ / 2) / 2)]], i, n + 1, (2 * n)) → list4
  10. CircleArc((0, 1 / (1 - m)), O_1, L) → b
  11. Segment((0, 0), (s, 0)) → d
  12. Midpoint(d) → B'
  13. CircleArc((0, 1), Q, (0, 0)) → c
  14. Segment(O, (x(O), y(O) - 1)) → e
  15. Midpoint(e) → G
  16. Circle((0, 1), 1) → g
  17. Midpoint(Q, O) → H
  18. Segment(O, A) → a
  19. Circle(P, 1, yAxis) → p
  20. Intersect(xOyPlane, p) → D, C
  21. Rotate(O_1, α, a) → F
  22. Surface((u * cos(v)), (u * sin(v)) + 1, f(u), u, 0, abs(x(D)), v, -1.5708, (3 * π / 2) - (2 * γ)) → o
  23. Rotate(D_1, α, a) → D'
  24. CircumcircleArc(D_1, C, D') → q
  25. Circle(O, D_1, a) → c_1
  26. Segment(P, D') → i
  27. Segment(P, D_1) → a_1
  28. Point(b) → J
  29. Segment(O, D_1) → b_1
  30. Vector(A + (0, 0, 0.3), A + (0, 0, 0.06)) → u
  31. Angle(D_1, O, A) → β