diff --git a/content/Variation of parameters (lec 9-10).md b/content/Variation of parameters (lec 9-10).md index ef19ae0..756cde6 100644 --- a/content/Variation of parameters (lec 9-10).md +++ b/content/Variation of parameters (lec 9-10).md @@ -94,9 +94,9 @@ $-2B\cos t+2A\sin(t)=2\cos t(t)$ $\implies A=0,\ B=-1$ $y_{p_{1}}=-\sin(t)$ $y''-2y'+y=e^t\ln(t)$ cant use undetermined coefficients, use variation of parameters -$y''_{p}(t)=v_{1}y_{1}+v_{2}y_{2}$ +$y_{p}''(t)=v_{1}y_{1}+v_{2}y_{2}$ $=v_{1}e^t+v_{2}te^t$ -Compute v1 and v2. This time lets do it using the linear system for funzies: +Compute $v_{1}$ and $v_{2}$. This time let's do it using the linear system for practice: eq1) $e^tv_{1}'+te^tv_{2}'=0$ eq2) $e^tv_{1}'+(te^t+e^t){v_{2}'}=e^t{\ln t}$ subtract eq1 from eq2: $v_{2}'=\ln(t)$ @@ -107,7 +107,7 @@ $=t\ln(t)-t$ no constant of integration. compute $v_{1}$ now: $v_{1}'=-tv_{2}'$ $=-t\ln t$ -integrate to get v_1: +integrate to get $v_1$: $v_{1}=-\int t\ln t \, dt$ integrate by parts (btw integration by parts will be the most important integration technique in this course): $v_{1}=-\frac{1}{2}(t^2\ln t)-\int t^2\frac{1}{t} \, dt$ @@ -115,5 +115,6 @@ $=-\frac{1}{2}\left( t^2\ln t-\frac{t^2}{2} \right)=-\frac{1}{2}t^2\ln t+\frac{1 $y_{p}''(t)=(\frac{1}{2}t^2\ln t+\frac{1}{4}t^2)e^t+(t\ln t-t)te^t$ $y_{p}(t)=-\sin(t)+\frac{1}{2}t^2\ln(t)e^t-\frac{3}{4}t^2e^t$ general solution is produced by adding the homogenous eq with $y_{p}(t)$ -general: -$$y(t)=c_{1}e^t+c_{2}te^t+y_{p}(t)$$ \ No newline at end of file +general solution: +$$y(t)=c_{1}e^t+c_{2}te^t-\sin(t)+\frac{1}{2}t^2\ln(t)e^t-\frac{3}{4}t^2e^t$$ +We are done. \ No newline at end of file