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单词 ProofOfJordansInequality
释义

proof of Jordan’s Inequality


To prove that

2πxsin(x)x,x[0,π2]

consider a circle (circle with radius = 1 ). Take any point P on the circumferenceMathworldPlanetmath of the circle.

Drop the perpendicularMathworldPlanetmathPlanetmathPlanetmath from P to the horizontal line, M being the foot of the perpendicular and Q the reflectionMathworldPlanetmath of P at M.(refer to figure)

Let x=POM.

For x to be in [0,π2], the point P lies in the first quadrantMathworldPlanetmath, as shown.

The length of line segmentMathworldPlanetmath PM is sin(x).Construct a circle of radius MP, with M as the center.

Length of line segment PQ is 2sin(x).

Length of arc PAQ is 2x.

Length of arc PBQ is πsin(x).

Since PQ length of arc PAQ (equality holds when x=0)we have 2sin(x)2x.This implies

sin(x)x

Since length of arc PAQ is length of arc PBQ (equality holds true when x=0 or x=π2),we have 2xπsin(x).This implies

2πxsin(x)

Thus we have

2πxsin(x)x,x[0,π2]
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更新时间:2025/5/4 12:09:40