2.9   ODE No. 9

\[ y'(x)-y(x) (a+\sin (\log (x))+\cos (\log (x)))=0 \] Mathematica : cpu = 0.0278232 (sec), leaf count = 19


\[\left \{\left \{y(x)\to c_1 e^{a x+x \sin (\log (x))}\right \}\right \}\] Maple : cpu = 0.025 (sec), leaf count = 14


\[y \relax (x ) = c_{1} {\mathrm e}^{x \left (\sin \left (\ln \relax (x )\right )+a \right )}\]

Hand solution

\begin {equation} \frac {dy}{dx}-y\relax (x) \left [ a+\sin \left (\log \relax (x) \right ) +\cos \left (\log \relax (x) \right ) \right ] =0\tag {1} \end {equation}

Integrating factor \(\mu =e^{-\int a-\sin \left (\log \relax (x) \right ) -\cos \left (\log \relax (x) \right ) dx}=e^{-ax}e^{-\int \sin \left ( \log \relax (x) \right ) +\cos \left (\log \relax (x) \right ) dx}\). To integrate \(\int \sin \left (\log \relax (x) \right ) +\cos \left (\log \relax (x) \right ) dx\), let \(r=\log \relax (x) \), \(\frac {dr}{dx}=\frac {1}{x}\), then \(dx=xdr\), But \(x=e^{r}\), hence the integral becomes

\begin {align} \int \sin \left (\log \relax (x) \right ) +\cos \left (\log \left ( x\right ) \right ) dx & =\int \left [ \sin \relax (r) +\cos \left ( r\right ) \right ] e^{r}dr\nonumber \\ & =\int e^{r}\sin \relax (r) dr+\int e^{r}\cos \relax (r) dr\tag {2} \end {align}

Integrating by parts \(\int e^{r}\cos \relax (r) dr,\) \(\int udv=uv-\int vdu\), Let \(u=e^{r}\rightarrow du=e^{r}\) and \(dv=\cos \relax (r) \rightarrow v=\sin \relax (r) \), hence (2) becomes

\begin {align*} \int e^{r}\sin \relax (r) dr+\int e^{r}\cos \relax (r) dr & =\int e^{r}\sin \relax (r) dr+e^{r}\sin \relax (r) -\int \sin \left ( r\right ) e^{r}dr\\ & =e^{r}\sin \relax (r) \end {align*}

Therefore, substituting back \(r=\log \relax (x) \) gives

\begin {align*} \int \sin \left (\log \relax (x) \right ) +\cos \left (\log \left ( x\right ) \right ) dx & =e^{\log \relax (x) }\sin \left (\log \left ( x\right ) \right ) \\ & =x\sin \left (\log \relax (x) \right ) \end {align*}

Hence the integration factor is

\begin {align*} \mu & =e^{-ax}e^{-\int \sin \left (\log \relax (x) \right ) +\cos \left ( \log \relax (x) \right ) dx}\\ & =e^{-ax}e^{-x\sin \left (\log \relax (x) \right ) } \end {align*}

Therefore (1) becomes

\[ \frac {d}{dx}\left (\mu y\relax (x) \right ) =0 \]

Integrating

\begin {align*} y\relax (x) e^{-ax}e^{-x\sin \left (\log \relax (x) \right ) } & =C\\ y\relax (x) & =Ce^{ax}e^{x\sin \left (\log \relax (x) \right ) }\\ & =Ce^{ax+x\sin \left (\log \relax (x) \right ) }\\ & =Ce^{x\left (a+\sin \left (\log \relax (x) \right ) \right ) } \end {align*}