Saturday, 16 November 2019

fine the limits :limxto0frac(sin2x2xcosx)(tan6x+tan(fracpi32x)tan(fracpi3+4x))xsinxtanxsin2x=?



fine the limits-without-lhopital rule and Taylor series :



lim




i know that :



\lim_{x \to 0} \frac{\sin x}{x}=1=\lim_{x \to 0}\frac{\tan x}{x}



But I can not answer please help .


Answer



If you know, that \enspace\displaystyle \lim\limits_{x\to 0}\frac{1}{x^2}(1-\frac{\sin x}{x})=\frac{1}{3!}\enspace then you can answer your question easily:



\displaystyle \frac{(\sin(2x)-2x\cos x)(\tan(6x)+\tan(\frac{\pi}{3}-2x)-\tan(\frac{\pi}{3}+4x))}{x\sin x\tan x\sin(2x)}=




\displaystyle =\frac{(\sin(2x)-2x\cos x)(\frac{\sin(6x)}{\cos(6x)}-\frac{\sin(6x)}{\cos(\frac{\pi}{3}-2x)\cos(\frac{\pi}{3}+4x)})}{x\sin x\tan x\sin(2x)}



\displaystyle =\frac{2\sin x\cos x -2x\cos x}{\sin x\tan x\sin(2x)}6\frac{\sin(6x)}{6x}(\frac{1}{\cos(6x)}-\frac{1}{\cos(\frac{\pi}{3}-2x)\cos(\frac{\pi}{3}+4x)})



\displaystyle =-\frac{1}{x^2}(1-\frac{\sin x}{x}) (\frac{x}{\sin x}\cos x)^2 \frac{2x}{\sin(2x)} 6\frac{\sin(6x)}{6x}(\frac{1}{\cos(6x)}-\frac{1}{\cos(\frac{\pi}{3}-2x)\cos(\frac{\pi}{3}+4x)})



\displaystyle \to -\frac{1}{3!}6(1-4)=3\enspace for \enspace x\to 0



A note about what I have used:




\displaystyle \tan x=\frac{\sin x}{\cos x}



\sin(2x)=2\sin x\cos x



\displaystyle \tan x-\tan y=\frac{\sin(x-y)}{\cos x\cos y}


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