Saturday, May 12, 2018

integration - Fundamental Theorem of Calculus for limlimitsxto0fracintx0(xt)sint2dtxsin3x


How to integrate this?




Evaluate lim



I've had difficulty in using L'Hopital rule. At the same time failed to really understand how to differentiate or evaluate the limit of the x\sin^3x and \int_0^x(x-t)\sin t^2\ dt


Would like to appreciate your help


Answer



Use repeatedly \sin u=u\,{\rm sinc}(u), whereby \lim_{u\to0}{\rm sinc}(u)={\rm sinc}(0)=1. We have \int_0^x (x-t)\sin(t^2)\>dt=x^4\int_0^1(1-\tau)\,\tau^2{\rm sinc}(x^2\tau^2)\>d\tau and \>x\sin^3 x=x^4\>{\rm sinc}^3(x)\ , so that {\int_0^x (x-t)\sin(t^2)\>dt \over x\sin^3 x}={\int_0^1(1-\tau)\,\tau^2\bigl(1+r(x,\tau)\bigr)\>d\tau\over 1+\bar r(x)}\ , whereby \lim_{x\to0}r(x,\tau)=0 uniformly in \tau, and \lim_{x\to0}\bar r(x)=0 as well. It follows that the limit in question is \int_0^1(1-\tau)\,\tau^2\>d\tau={1\over12}\ .


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