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91Ó°ÊÓ

find a general solution to the given equation.

y'''+3y''-4y=e-2x

Short Answer

Expert verified

y(x)=-16x2e-2x+c1e-2x+c2xe-2x+c3ex

Step by step solution

01

Find the corresponding auxiliaryequation

Theauxiliary equationof corresponding homogeneous equation

r3+3r2-4=(r-1)(r+2)2=0

The solutions of the auxiliary equation are

r=-2,r=-2,r=1

Therefore a general solution to the homogeneous equation is

yh(x)=c1e-2x+c2xe-2x+c3ex

02

Find particular solution

Let the particular solution be

yp(x)=ax2e-2x

Then

yp'(x)=2axe-2x-2ax2e-2xyp''(x)=2ae-2x-8axe-2x+4ax2e-2xyp'''(x)=-12ae-2x+24axe-2x-8ax2e-2x

Then

yp'''(x)+3yp''(x)-4yp(x)=-12ae-2x+24axe-2x-8ax2e-2x+6ae-2x-24axe-2x+12ax2e-2x-4ax2e-2x=-6ae-2x.

If-6ae-2x=e-2xthen6a=1

Thena=-16

Hence

yp(x)=-16x2e-2x

03

Step 3: y(x)=yh+yp

Theny(x)=-16x2e-2x+c1e-2x+c2xe-2x+c3ex

Is the general solution ofy'''+3y''-4y=e-2x

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