Find inverse Laplace transform of : $\ln(\frac{s^2+a^2}{s^2+b^2})$ Question :

Find inverse Laplace transform of :
$$\ln \left(\frac{s^2+a^2}{s^2+b^2}\right)$$

My try :
I'm trying use this identity :
$f(t)=-\frac{\mathcal{L}^{-1}(\frac{dF(s)}{ds})}{t}$
Let $F(s)=\ln(\frac{s^2+a^2}{s^2+b^2})$
Then :
$\frac{dF(s)}{ds}=\frac{2s}{s^2+a^2}-\frac{2s}{s^2+b^2}$
So : $-tf(t)=\mathcal{L}^{-1}(\frac{2s}{s^2+a^2}-\frac{2s}{s^2+b^2})$
$=2\cos (at)-2\cos (bt)$
Is my try correct ?
If any one have simple method plz help me
 A: Using the change of variable $s=\frac{1}{x}$
$$F\left( s \right)=\ln \left( \frac{{{s}^{2}}+{{a}^{2}}}{{{s}^{2}}+{{b}^{2}}} \right)=\ln \left( \frac{1+{{a}^{2}}{{x}^{2}}}{1+{{b}^{2}}{{x}^{2}}} \right)=\ln \left( 1+{{a}^{2}}{{x}^{2}} \right)-\ln \left( 1+{{b}^{2}}{{x}^{2}} \right)$$
and the power series expansion:
$$\ln \left( 1+u \right)=u-\frac{{{u}^{2}}}{2}+\frac{{{u}^{3}}}{3}-\cdots =\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}}{n}{{u}^{n}}}$$
hence
$$F\left( s \right)=\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}}{n}{{\left( \frac{a}{s} \right)}^{2n}}-}\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}}{n}{{\left( \frac{b}{s} \right)}^{2n}}}$$
or
$$f\left( t \right)=\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}{{a}^{2n}}}{n}{{\mathcal{L}}^{-1}}{{\left( \frac{1}{s} \right)}^{2n}}-}\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}{{b}^{2n}}}{n}{{\mathcal{L}}^{-1}}{{\left( \frac{1}{s} \right)}^{2n}}}$$
so
$$f\left( t \right)=\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}{{a}^{2n}}}{n}\frac{{{t}^{2n-1}}}{\left( 2n-1 \right)!}-}\sum\nolimits_{n=1}^{\infty }{\frac{{{\left( -1 \right)}^{n+1}}{{b}^{2n}}}{n}\frac{{{t}^{2n-1}}}{\left( 2n-1 \right)!}}$$
at last
$$\begin{align}
  & \frac{1}{2}tf\left( t \right)=\sum\nolimits_{n=1}^{\infty }{{{\left( -1 \right)}^{n+1}}\frac{{{\left( at \right)}^{2n}}}{\left( 2n \right)!}-}\sum\nolimits_{n=1}^{\infty }{{{\left( -1 \right)}^{n+1}}\frac{{{\left( bt \right)}^{2n}}}{\left( 2n \right)!}} \\ 
 & \quad \quad \quad =\left( 1-\cos \left( at \right) \right)-\left( 1-\cos \left( bt \right) \right) \\ 
\end{align}$$
and you get
$$f\left( t \right)=\frac{2\cos \left( bt \right)-2\cos \left( at \right)}{t}$$
