Constructive Triangles: Second Box: Heads and Tails

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Box 2 -

  • Two blue equilateral triangles
  • Two blue right-angled isosceles triangles
  • Two blue right-angled scalene triangles
  • One blue obtuse-angled scalene triangle
  • One blue right-angled scalene triangle (corresponds to the red triangle from Box 1)



  1. Every plane figure, like a coin, has two sides.
  2. One side is called obverse or heads; it is the side which has a face or the principle design.
  3. When you turn it over, you have the reverse side or tails.
  4. All of these figures have an obverse (blue) and a reverse (natural wood or white) side.
  5. Isolate the two equilateral triangles.
  6. Invite the child to form as many figures as possible.
  7. As before, he can make only one.
  8. Suggest that he tries with one obverse and one reverse side.
  9. It won't help. There is only one figure he can make.
  10. With the two isosceles triangles he makes the two possible quadrilaterals with the obverse sides.
  11. Invite the child to make a triangle.
  12. The child classifies the triangle: isosceles.
  13.  By turning one triangle to its reverse side, the child can make no new figures.
  14. With the two scalene triangles, the child tries to form all possible quadrilaterals first with the two obverse sides (yielding the same three figures as before) and then with one reverse side.
  15. The child is able to form a new figure: a kite (or a deltoid, having the form of the Greek capital letter Delta ).
  16. Invite the child to make triangles, first with obverse sides (yielding none) and then with one reverse side.
  17. The child classifies the triangles he makes: acute-angled isosceles and obtuse-angled isosceles triangles.

Control Of Error

Points Of Interest




Constructive Triangles Boxes # 1-5