Mathematics on the blackboard: A fine and quite rare theme on stamps. The teacher explains classical theorems in Euclidean plane geometry from Euclid's Elements: The inscribed angle theorem which is covered by two seperate theorems in the Elements.
At the top of the stamp we read in Portuguese International Year of the Child 1979.
Inscribed angle theorem, part 1 Euclid Elements Book 3, Proposition 20
We draw a triangle in a circle with a chord as a base and the center of the circle as opposite vertex (point O on the stamp). We inscribe another triangle with the same base and the opposite vertex (point A on the stamp) on the arc which lies on the same side as O . The angle of the triangle at O is twice the angle at A ("centerangle=2·inscribedangle").
Inscribed angle theorem, part 2 Euclid Elements Book 3, Proposition 21
All inscribed triangles as constructed in part 1 have equal inscribed angles in A . The position of A has no effect on the angle at A .
Quite often, the two theorems are proved by first proving a special case (e.g. source). But this was not the way Euclid constructed the proof. More than 2300 years ago, he gave a simple and elegant proof in the Elements; it is illustrated in figure 1. This proof assumedly goes back to the Pythagoreans who worked on geometry 2500 years ago.
In figure 1 we find other labellings as in the stamp. The vertices in the center and on the arc are M and C . They are joined by a straight line. AB is the base of the triangles. AMC und BMC are isosceles triangles.
δ1+φ1 = 180° and
δ1+2γ1 = 180° ⇒ φ1 = 2γ1
In the same way we get φ2=2γ2 , so φ=2γ . This derivation makes no use of the position of C , so both theorems are proved.
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