Question #122593
1. Solve the given differential equation by undetermined coefficients.
y'' − 2y' + 5y = e^x sin x

y(x) =____

2. Solve the given differential equation by undetermined coefficients.
y'' − y' − 2y = e^2x

y(x) =___
1
Expert's answer
2020-06-30T18:01:02-0400

1.The given equation is (D22D+5)y=exsinx(D^{2}-2D+5)y = e^{x}\sin x.

The auxiliary equation is m22m+5=0.m^{2}-2m+5=0. Solving for m, we get m=1±2im=1\pm 2i.

Thus, the complementary function is C.F=ex(c1cos2x+c2sin2x).C.F = e^{x}(c_{1}\cos 2x + c_{2}\sin 2x).


To find the particular integral, let yp=Aexsinxy_{p} = Ae^{x}\sin x be the trial solution.

Then,

yp=Aex(cosx+sinx) and yp=A(ex(sinx+cosx)+ex(cosx+sinx))     =2Aexcosxy'_{p} = Ae^{x}(\cos x + \sin x)~\text{and}~\\ y''_{p} = A\bigg(e^{x}(-\sin x+\cos x )+e^{x}(\cos x + \sin x)\bigg)\\ ~~~~~= 2Ae^{x}\cos x


Therefore,

2Aexcosx2(Aex(cosx+sinx))+5Aexsinx=exsinxA=132Ae^{x}\cos x - 2\bigg(Ae^{x}(\cos x+\sin x)\bigg)+5Ae^{x}\sin x = e^{x}\sin x\\ A = \dfrac{1}{3}

Hence the particular integral is yp=13exsinxy_{p} = \dfrac{1}{3}e^{x}\sin x .

The general solution is y=ex(c1cos2x+c2sin2x)+13exsinxy=e^{x}(c_{1}\cos 2x+c_{2}\sin 2x)+\dfrac{1}{3}e^{x}\sin x


2.The given equation is (D2D2)y=e2x.(D^{2}-D-2)y=e^{2x}.

The auxiliary equation is m2m2=0m^{2}-m-2=0. Solving for m, we get m=1,2m=-1,2.

Thus, the complementary function is C.F=c1ex+c2e2xC.F=c_{1}e^{-x}+c_{2}e^{2x}.


To find the particular integral, since g(x)=e2xg(x) =e^{2x} is a linear combination of a term in complementary function we choose yp=Axe2xy_{p} = Axe^{2x} as the trial particular solution.

Thus,

yp=A(2xex+e2x)=Ae2x(2x+1)yp=A(2(2xex+e2x)+2e2x)=4Ae2x(x+1)y'_{p}=A(2xe^{x}+e^{2x})=Ae^{2x}(2x+1)\\ y''_{p}=A\bigg(2(2xe^{x}+e^{2x})+2e^{2x}\bigg)=4Ae^{2x}(x+1)\\


Therefore,

4Ae2x(x+1)Ae2x(2x+1)2Axe2x=e2xA(4x+42x12x)=1A=134Ae^{2x}(x+1)-Ae^{2x}(2x+1)-2Axe^{2x}=e^{2x}\\ A(4x+4-2x-1-2x)=1\\ A=\dfrac{1}{3}

Hence the particular integral is, yp=13xe2xy_{p}=\dfrac{1}{3}xe^{2x}


The general solution is, y=c1ex+e2x(x3+c2)y=c_{1}e^{-x}+e^{2x}(\frac{x}{3}+c_{2})


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