I came across an integral that is to be solved by the Laplace transform method.
$$ \int_0^{\infty} \frac{\sin^2{x}}{x^2}$$
My Approch
let $ f(t) = \sin^{2}t$ and $$\sin^{2}t = \frac{1-\cos2t}{2}$$
also we know from the properties of Laplace transforms
$$\frac{\mathcal{L}{f(t)}}{t} = \int_s^\infty F(s) ds $$
so $$ \mathcal{L}\left[\frac{1-\cos2t}{2}\right] = \frac{1}{2} \left[ \frac{1}{s} – \frac{s}{s^2+4} \right]$$
now, $$\mathcal{L}\left[\frac{1-\cos2t}{2t}\right] = \frac{1}{2}\int_s^\infty \left(\frac{1}{s} -\frac{s}{s^2+4} \right) ds$$
after integrating
$$ \mathcal{L} \left[\frac{1-cos2t}{2t}\right]= \frac{1}{2}\left[\ln(s) – \frac{\ln(s^2+4)}{2}\right]\big{|}_s^\infty$$
$$ \mathcal{L}\left[\frac{1-\cos2t}{2t}\right] =\frac{1}{2} \left[\ln(\frac{s}{(s^2+4)^{1/2}})\right] \big{|}_s^\infty $$
but at this point the integral becomes not-defined,
How can I approach this question as it has to be solved by the method of Laplace transforms
Hints are appreciated
Evaluating the integral with help of Laplace transforms
integrationlaplace transform
Best Answer
First, there's a missing $2$ in the denominator of $\frac{1-\cos 2t}{t}$. I suppose it's a typo? Next, $$\mathcal{L}\left[\frac{1-\cos2t}{2t}\right] = \frac{1}{2}\int_0^\infty \left(\frac{1}{s} -\frac{s}{s^2+4} \right) ds $$ can't be true because the RHS is possibly a number (the integral actually diverges) while the LHS is a function. What we have instead is $$\mathcal{L}\left[\frac{\sin^2 t}{t}\right]=\mathcal{L}\left[\frac{1-\cos2t}{2t}\right]=\frac 12\int_s^{\infty}\left(\frac 1y-\frac{y}{y^2+4}\right)dy=\frac 14\log\frac{s^2+4}{s^2} $$ Now, if we let $g(t)=\sin^2 t/t$, we've found $\mathcal{L}[g(t)]$. So $$\int_0^{\infty}\frac{\sin^2 x}{x^2}dx=\int_0^{\infty}\frac{g(x)}{x}dx=\frac 14\int_0^{\infty}\log\frac{p^2+4}{p^2}dp $$ Can you continue from here? Use integration by parts.