Sunday, 13 July 2014

calculus - Is the proof of limthetato0fracsinthetatheta=1 in some high school textbooks circular?

I was taught the following proof in high school.




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By constructing triangles with 0<θ<π/2 and a circle with radius r and by comparing the areas, we have
12r2sinθcosθ12r2θ12r2tanθ


Hence
cosθθsinθ1cosθ

Then by squeeze theorem, we have the result.



My question is, the middle term in the above inequality comes from the fact that the area of the circle is πr2, which in my textbooks, is later proved by integration. But the integration requires results in calculus which comes from the fact that
limθ0sinθθ=1

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