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@@ -2564,8 +2564,6 @@ <h4>Archimedes and the Illusion of Limits</h4>
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His polygon method for estimating the pi is often celebrated as a triumph of geometric reasoning.
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But the pi, as obtained by that method, is not a Euclidean constant — it is an analytic approximation derived from limits.
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And the method itself quietly imports assumptions that elementary geometry never provided.
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Archimedes approximated the circumference of a circle using inscribed and circumscribed polygons.
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Observing that the perimeters of the inscribed and circumscribed polygons approached one another, he concluded that their common limit must be the circumference.
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To compute these perimeters, Archimedes relied on straight‑line geometry expressed in terms of the sine and cosine of the polygon angles.
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And here is the crucial point:
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To compute these perimeters, Archimedes relied on straight‑line geometry expressed in terms of the sine and cosine of the polygon angles.
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Pure Euclidean construction gives exact ratios only for a very small set of angles:</p>
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Pure geometric construction gives exact ratios for a set of angles:</p>
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<ulstyle="margin:6px">
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<li>90°</li>
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<li>60°</li>
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<li>45°</li>
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<li>30°</li>
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<li>22.5°</li>
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<li>15°</li>
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<li>7.5°</li>
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</ul>
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<p>These arise from:</p>
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<li>the 30–60–90 triangle°</li>
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</ul>
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<p>For these angles, the familiar identity
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<p>For these angles, the identity
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sin(2x) = 2sin(x)cos(x)
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is not a deep theorem — it is simply a geometric tautology arising from symmetry.
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is a geometric tautology arising from symmetry.
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The angles 22.5°, 15° and 7.5° can be constructed by subtracting a triangle from a known triangle, but Euclid stops there.
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The angles 22.5°, 15°, 7.5° and 3.75° can be constructed by subtracting a triangle from a known triangle.
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Euclid gives exact angle bisection as a geometric construction, but he provides no computational framework for evaluating the sine of the bisected angles.
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Any angle can be bisected by compasses and straightedge, yet — with a few exceptions — their numerical values cannot be derived from Euclid’s axioms or propositions.
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Any method for computing these values requires additional, non‑Euclidean assumptions, that were added centuries later:</p>
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<ulstyle="margin:6px">
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<li>the angle‑addition formulas</li>
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<li>the half‑angle formulas</li>
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<li>the continuity of sine</li>
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<li>the differentiability of sine</li>
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<li>the analytic extension of sine to all real numbers</li>
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</ul>
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<p>None of these appear in Euclid’s Elements.</p>
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<p>The classical half‑angle formula</p>
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The classical half‑angle formula</p>
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<mathxmlns="http://www.w3.org/1998/Math/MathML">
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<mrow>
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</msqrt>
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</mrow>
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</math>
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<p>is not a Euclidean theorem.
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It is a theorem of analytic trigonometry, which presupposes the angle‑addition formulas and treats sine and cosine as smooth analytic functions.
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The classical bisection formulas assume the identity
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sin(2x)=2sin(x)cos(x)
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for all x.
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But this identity is not derivable from Euclid. It is an axiom of the analytic system.
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It holds for the Euclidean angles 90°, 60°, 45°, and 30° because those triangles are special.
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There is no geometric guarantee that it holds for arbitrary angles.
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Thus, when Archimedes computed sin(3.75°) he was not using Euclid.
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He was using a numerical trigonometric ladder built on analytic assumptions that Euclid never supplied.
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If the angle‑bisection formulas are slightly inaccurate for non‑special angles, then by the time Archimedes reached the 96‑gon, that error had compounded — even though the construction of the angles themselves was exact.
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<p>enables exact calculations for the sides of the polygons.
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But even if we momentarily accept these analytic assumptions, a deeper geometric inconsistency emerges when we compare circumscribed polygons to circles.
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But a geometric inconsistency emerges when we compare circumscribed polygons to circles.
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A “circumscribed polygon” is not guaranteed to be an upper bound in every sense.
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