《微积分:一元函数积分学》——三角函数积分进阶

前言

对于三角函数相关的积分问题,往往需要对三角函数进行恒等变形,通常要借助如下这些公式

1、倍角公式

(1)sin2a=2sinacosa

(2)cos2a=cos^{2}a-sin^{2}a=1-2sin^{2}a=2cos^{2}a-1

(3)sin3a=-4sin^{3}a+3sina

(4)cos3a=4cos^{3}a-3cosa

(5)tan2a=\frac{2tana}{1-tan^{2}a}

(6)cot2a=\frac{cot^{2}a-1}{2cota}

2、半角公式

(1)sin^{2}\frac{a}{2}=\frac{1-cosa}{2}(降幂公式)

(2)cos^{2}\frac{a}{2}=\frac{1+cosa}{2}(降幂公式)

(3)tan\frac{a}{2}=\frac{1-cosa}{sina}=\frac{sina}{1+cosa}=\pm \sqrt{\frac{1-cosa}{1+cosa}}

(3)cot\frac{a}{2}=\frac{sina}{1-cosa}=\frac{1+cosa}{sina}=\pm \sqrt{\frac{1+cosa}{1-cosa}}

3、和差公式

(1)sin(\alpha \pm \beta )=sin\alpha cos\beta \pm cos\alpha sin\beta

(2)cos(\alpha \pm \beta )=cos\alpha cos\beta \mp sin\alpha sin\beta

(3)tan(\alpha \pm \beta )=\frac{tan\alpha\pm tan\beta}{1\mp tan\alpha tan\beta}

(4)cot(\alpha \pm \beta )=\frac{cot\alpha cot\beta\mp 1}{cot\beta\pm cot\alpha}

4、积化和差公式

(1)sin\alpha cos\beta =\frac{1}{2}[sin(\alpha + \beta)+sin(\alpha - \beta)]

(2)cos\alpha sin\beta =\frac{1}{2}[sin(\alpha + \beta)-sin(\alpha - \beta)]

(3)cos\alpha cos\beta =\frac{1}{2}[cos(\alpha + \beta)+cos(\alpha - \beta)]

(2)sin\alpha sin\beta =\frac{1}{2}[cos(\alpha - \beta)-cos(\alpha + \beta)]

5、和差化积公式

(1)sin\alpha +sin\beta=2sin\frac{\alpha+\beta}{2}cos\frac{\alpha-\beta}{2}

(2)sin\alpha -sin\beta=2sin\frac{\alpha-\beta}{2}cos\frac{\alpha+\beta}{2}

(3)cos\alpha +cos\beta=2cos\frac{\alpha+\beta}{2}cos\frac{\alpha-\beta}{2}

(4)cos\alpha-cos\beta=-2sin\frac{\alpha+\beta}{2}sin\frac{\alpha-\beta}{2}

6、万能公式

 \\\\u=tan\frac{x}{2} \quad(-\pi<x<\pi) \\\\\Rightarrow sinx=\frac{2u}{1+u^{2}}\quad cosx=\frac{1-u^{2}}{1+u^{2}}\quad du=\frac{2}{1+u^{2}}du

积分列表

(1)\int sin^{n}dx

(2)\int cos^{n}xdx

(3)\int \frac{dx}{sin^{n}x}

(4)\int\frac{dx}{cos^{n}x}

(5)\int cos^{m}xsin^{n}xdx

(6)\int sinaxcosbxdx

(7)\int sinaxsinbxdx

(8)\int cosaxcosbxdx

(9)\int\frac{dx}{a+bsinx} \quad(a^{2}>b^{2})

(10)\int\frac{dx}{a+bsinx} \quad(a^{2}<b^{2})

(11)\int\frac{dx}{a+bcosx} \quad(a^{2}>b^{2})

(12)\int\frac{dx}{a+bcosx} \quad(a^{2}<b^{2})

(13)\int\frac{dx}{a^{2}cos^{2}x+b^{2}sin^{2}x}

(14)\int\frac{dx}{a^{2}cos^{2}x-b^{2}sin^{2}x}

(15)\int xsinaxdx

(16)\int x^{2}sinaxdx

(17)\int xcosaxdx

(18)\int x^{2}cosaxdx

 

实例

(1)\int\frac{dx}{sin2x+2sinx}

思路:努力将sinx和cosx化到tan(x/2)

\\\\\\\int\frac{dx}{sin2x+2sinx} \\\\\\=\int\frac{dx}{2sinx(cosx+1)} \\\\\\=\frac{1}{4}\int\frac{d(\frac{x}{2})}{sin\frac{x}{2}cos^{3}\frac{x}{2}} \\\\\\=\frac{1}{4}\int\frac{d(tan\frac{x}{2})}{tan\frac{x}{2}cos^{2}\frac{x}{2}} \\\\\\=\frac{1}{4}\int\frac{1+tan^{2}\frac{x}{2}}{tan\frac{x}{2}}d(tan\frac{x}{2}) \\\\\\=\frac{1}{8}tan^{2}\frac{x}{2}+\frac{1}{4}ln\left | tan\frac{x}{2} \right |+C

 

 (2)\int\frac{dx}{1+sinx}

思路一 (推荐)

《微积分:一元函数积分学》——三角函数积分进阶_第1张图片

思路二:努力将sinx化到tan(x/2)

\\\\\\\int\frac{dx}{(cos\frac{x}{2}+sin\frac{x}{2})^{2}} \\\\\\=\int\frac{sec^{2}\frac{x}{2}}{(1+tan\frac{x}{2})^{2}}dx \\\\\\=2\int\frac{d(1+tan\frac{x}{2})}{(1+tan\frac{x}{2})^{2}} \\\\\\=-\frac{2}{1+tan\frac{x}{2}}+C

 

(3)\int\frac{dx}{1+sinx+cosx} 

思路:努力将sinx和cosx化到tan(x/2)

\\\\\\\int\frac{dx}{1+sinx+cosx} \\\\\\=\int\frac{dx}{2sin\frac{x}{2}cos\frac{x}{2}+2cos^{2}\frac{x}{2}} \\\\\\=\frac{1}{2}\int\frac{dx}{cos^{2}\frac{x}{2}(1+tan\frac{x}{2})} \\\\\\=\int\frac{d(1+tan\frac{x}{2})}{1+tan\frac{x}{2}} \\\\\\=ln\left | 1+tan\frac{x}{2} \right |+C

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