Question #125360
Use the method of variation of parameters to determine the general solution of the given differential equation.

y′′′−y′=5t

Use C1, C2, C3, ... for the constants of integration.

y(t) = _______
1
Expert's answer
2020-07-07T19:28:49-0400

The given differential equation can be written as (D3D)y=5t, where Dddt.(D^{3}-D)y = 5t,~ \text{where}~ D\equiv \dfrac{d}{dt}.


The auxiliary equation is, m3m=0.m^{3}-m=0. Solving, we get m=0,1,1.m = 0,1,-1.


The complementary function is, yc=c1+c2et+c3et=c1f1+c2f2+c3f3y_{c} = c_{1}+c_{2}e^{t}+c_{3}e^{-t} = c_{1}f_{1}+c_{2}f_{2}+c_{3}f_{3}


The particular integral is yp=Pf1+Qf2+Rf3,y_{p}=Pf_{1}+Qf_{2}+Rf_{3}, where


P=W1W(f1,f2,f3)dx;Q=W2W(f1,f2,f3)dx; R=W3W(f1,f2,f3)dx;P=\displaystyle\int \frac{W_{1}}{W(f_{1},f_{2},f_{3})}dx; Q=\displaystyle\int \frac{W_{2}}{W(f_{1},f_{2},f_{3})}dx;\\~\\ R=\displaystyle\int \frac{W_{3}}{W(f_{1},f_{2},f_{3})}dx;


and WiW_{i} is the determinant obtained from the Wronskian W(f1,f2,f3)W(f_{1},f_{2},f_{3}) by replacing the ithi^{th} - column by the vector (00g(t))\begin{pmatrix} 0 \\ 0 \\ g(t) \end{pmatrix} where g(t)=5t.g(t) = 5t.

Now,


W(f1,f2,f3)=1etet0etet0etet=2 W1=0etet0etet5tetet=10t W2=10et00et05tet=5tet W3=1et00et00et5t=5tetW(f_{1},f_{2},f_{3}) = \begin{vmatrix} 1 & e^{t} & e^{- t} \\ 0 & e^{t} &- e^{- t} \\0 & e^{t} & e^{- t} \end{vmatrix} = 2\\~\\ W_{1} = \begin{vmatrix} 0 & e^{t} & e^{- t} \\ 0 & e^{t} &- e^{- t} \\5t & e^{t} & e^{- t} \end{vmatrix} = -10t\\~\\ W_{2} = \begin{vmatrix} 1 & 0 & e^{- t} \\ 0 & 0 &- e^{- t} \\0 & 5t & e^{- t} \end{vmatrix} = 5te^{-t}\\~\\ W_{3} = \begin{vmatrix} 1 & e^{t} & 0 \\ 0 & e^{t} & 0 \\0 & e^{t} &5t \end{vmatrix} = 5te^{t}


Therefore,


P=5t dt=5t22 Q=52tet dt=52(tetet)=52et(t+1) R=52tet dt=52(tetet)=52et(t1)P = \displaystyle\int -5t~ dt = -\frac{5t^{2}}{2}\\~\\ Q= \displaystyle \frac{5}{2}\int te^{-t}~ dt = \frac{5}{2}(-te^{-t}-e^{-t}) = -\frac{5}{2}e^{-t}(t+1)\\~\\ R= \displaystyle \frac{5}{2}\int te^{t}~ dt = \frac{5}{2}(te^{t}-e^{t}) = \frac{5}{2}e^{t}(t-1)


Hence,


yp=Pf1+Qf2+Rf3=5t2252et(t+1)et+52et(t1)et=5t2252(t+1)+52(t1)=5t225\begin{aligned} y_{p} &=Pf_{1}+Qf_{2}+Rf_{3}\\ &=-\frac{5t^{2}}{2}-\frac{5}{2}e^{-t}(t+1) \cdot e^{t} + \frac{5}{2}e^{t}(t-1) \cdot e^{-t}\\ &=-\frac{5t^{2}}{2}-\frac{5}{2}(t+1) + \frac{5}{2}(t-1)\\ &=-\frac{5t^{2}}{2} - 5 \end{aligned}


The general solution is

y=yc+yp=c1+c2et+c3et5t225y=C1+c2et+c3et5t22,where C1=c15.\begin{aligned} y = y_{c}+y_{p} &= c_{1}+c_{2}e^{t}+c_{3}e^{-t}-\dfrac{5t^{2}}{2}-5\\ y&= C_{1}+c_{2}e^{t}+c_{3}e^{-t}-\dfrac{5t^{2}}{2}, \text{where}~C_{1}=c_{1}-5. \end{aligned}


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