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Q38E

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Fundamentals Of Differential Equations And Boundary Value Problems
Found in: Page 338
Fundamentals Of Differential Equations And Boundary Value Problems

Fundamentals Of Differential Equations And Boundary Value Problems

Book edition 9th
Author(s) R. Kent Nagle, Edward B. Saff, Arthur David Snider
Pages 616 pages
ISBN 9780321977069

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Short Answer

In Problems 38 and 39, use the elimination method of Section to find a general solution to the given system.

x-d2y/dt2=t+1

dx/dt+dy/dt-2y=et

The general solution is x(t)=-3c1-7c22e-t2cos72t+7c1-3c22e-t2sin72t+c3+12et+14tet+t+1y(t)=c1e-t2cos72t+c2e-t2sin72t+c3et+14tet+1

See the step by step solution

Step by Step Solution

Step 1: Definition

A differential equation is an equation that contains one or more functions with its derivatives.

Step 2: Simplify equation

It is given that x-y''=t+1

x=y''+t+1-------(1)

Differentiating equation (1) we have:

x'=y'''+1

Substituting value we get:

x'+y'-2y=ety'''+y'-2y+1=e'

Step 3: For general solution

The auxiliary equation is given by:

D3+D-2y=et-1

The homogenous equation is D3+D-2=0:

D=1,-12±i72

So, the general solution is given by yg=C1et+C2e-t2cos72t+C3e-t2sin7t2

Step 4: For particular solution

Let yp=Atet+B

Differentiating we have:

yp'=Aet(t+1)yp''=Aet(t+2)

yp'''=Aet(t+3)yp'''+yp'-2yp=et-1

Substituting value & comparing we get:

Aet(t+3+t+1-2t)-2B=et-14A=1,-2B=-1A=14,B=12

So y=C1et+C2e-t2cos72t+C3e-t2sin72t+tet4+12

Step 5: Compute value x

Now we need to substitute value of y''in x=y''+t+1.

y'=C1e'-C22e-t2cos7t2-72C2e-t2sin7t2+72C3e-t2cos7t2-C32e-t2sin7t2+et4(t+1)y'=e-t2cos7t27C3-C22+e-t2sin7t2-7C2-C32+ett4+C1+14

And

y''=e-t2sin7t2-C2+C372-72-12et2cos7t27C3-C22-7C2+C3272e-t2cos7t2-12e-t2cos7t2+ett4+C1+12y''=e-t2sin7t27C2-3C34+e-t2cos7t2-3C2-7C32+ett4+C1+12

Substituting values we get:

x=e-t2sin7t27C2-3C34+e-t2cos7t2-3C2-7C32+ett4+C1+12+t+1

Hence the solution of equation is given by

x=e-t2sin7t27C2-3C34+e-t2cos7t2-3C2-7C32+ett4+C1+12+t+1

Therefore the solution is :

x(t)=-3c1-7c22e-t2cos72t+7c1-3c22e-t2sin72t+c3+12et+14tet+t+1y(t)=c1e-t2cos72t+c2e-t2sin72t+c3et+14tet+1

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