数学中有各式各样富含诗意的曲线,螺旋线就是其中比较特别的一类.螺旋线这个名词来源于希腊文,它的原意是“旋卷”或“缠卷”.小明对螺旋线有着浓厚的兴趣,用以下方法画出了如图所示的螺旋线.具体作法是:先作边长为1的正三角形ABC,分别记射线AC,BA,CB为l
1 , l
2 , l
3 , 以C为圆心、CB为半径作劣弧BC
1交l
1于点C
1;以A为圆心、AC
1为半径作劣弧C
1A
1交l
2于点A;以B为圆心、BA
1为半径作劣弧A
1B
1交l
3于点B
1 , …,依此规律作下去,就得到了一系列圆弧形成的螺旋线.记劣弧BC
1的长,劣弧C
1A
1的长,劣弧A
1B
1的长,…依次为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E1%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E++%EF%BC%8C+%3C%2Fmo%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E2%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E++%EF%BC%8C+%3C%2Fmo%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E3%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E++%EF%BC%8C+%3C%2Fmo%3E%3Cmo%3E%E2%8B%AF%3C%2Fmo%3E%3Cmo%3E++%EF%BC%8C+%3C%2Fmo%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
则
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E1%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%2B%3C%2Fmo%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E2%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%2B%3C%2Fmo%3E%3Cmo%3E%E2%8B%AF%3C%2Fmo%3E%3Cmo%3E%2B%3C%2Fmo%3E%3Cmsub%3E%3Cmi%3Ea%3C%2Fmi%3E%3Cmn%3E9%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%3D%3C%2Fmo%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
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