Скачать или смотреть Calculus 1 — 6.6: The Three Essential Trig Limits

Why does sin(x)/x equal exactly 1 as x approaches 0 — and why can't you use L'Hôpital's Rule to prove it? This video builds the proof from scratch using the unit circle and the Squeeze Theorem, then derives two more foundational trig limits and shows how to apply them to increasingly complex problems.

Key concepts covered:
• Why sin(x)/x → 1 as x → 0 cannot be proven with L'Hôpital's Rule (circular reasoning)
• Geometric proof using three nested regions on the unit circle: inscribed triangle, circular sector, and tangent triangle
• Deriving the bounding inequality: cos(x) ≤ sin(x)/x ≤ 1
• The Squeeze Theorem (Sandwich Theorem): trapping a function between two converging bounds
• Second essential limit: (1 − cos x)/x → 0 via conjugate multiplication and the Pythagorean identity
• Third essential limit: tan(x)/x → 1 by rewriting as [sin(x)/x] · [1/cos(x)]
• The Matching Principle: ensuring the trig function's argument equals the denominator
• Worked example: sin(2x)/x = 2 by multiplying and dividing to create a matching form
• Worked example: sin(5x)/sin(6x) = 5/6 using the general pattern sin(ax)/sin(bx) = a/b
• Taming wild oscillations: proving x·sin(1/x) → 0 by bounding between −|x| and |x|

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SOURCE MATERIALS
The source materials for this video are from    • Calculus 1 Lecture 1.2:  Properties of Lim...