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Fundamentals Of Differential Equations And Boundary Value Problems
Found in: Page 391
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 25 - 32, solve the given initial value problem using the method of Laplace transforms.

y''+5y'+6y=g(t):y(0)=0,y'(0)=2, where g(t)={0 , 0t<1,t , 1<t<5,1 ,5<t

The solution of the given initial value problem using the method of Laplace transforms is

y(t)=2e-2t-2e-3t+136+16(t-1)-14e-2(t-1)+29e-3(t-1)u(t-1)-1936+16(t-5)-74e-2(t-5)+119e-3(t-5)u(t-5)

See the step by step solution

Step by Step Solution

Step 1: Define Laplace Transform

The use of Laplace transformation is to convert differential equations into algebraic equations.T

F(s)=0+f(t)·e-s·tdt

Where, F(s)= Laplace transformation

S= Complex number

t= real number>=0

t'= first derivative of teh function f(t)

Step 2: Apply Laplace transform:

Given initial value problem

y''+5y'+6y=g(t)

where Also

g(t)=t,1<t<5=1,5<t

Using rectangular and unit function we can write

g(t)=t[u(t-1)-u(t-5)]+1u(t-5)=tu(t-1)-(t-1)u(t-5)y(0)=0 and y'(0)=2

Taking Laplace transform of initial value problem is

Ly''(s)+5Ly'(s)+6Ly(s)=L[g(t)]

s2Ly(s)-sLy(0)-y'(0)+5sLy(s)-5y(0)+6y(s)=L[tu(t-1)-(t-1)u(t-5)]s2Ly(s)+0-2+5sLy(s)-0+6Ly(s)=e-81s2+1s-e-5s1s2+4ss2+5s+6Ly(s)-2=e-8(1+s)s2-e-5s(1+4s)s2

Ly(s)=2s2+5s+6+e-A(1+s)s2s2+5s+6-e-5s(1+1s)s2s2+5s+6=2s+2-2s+3+es(1+s)s2s2+5s+6-e5s(1+4s)s2s2+5s+6(1)

Using partial fraction

1+ss2s2+5s+6=136s+16s2-14s+2+29s+31+4ss2s2+5s+6=1936s+16s2-74s+2+119s+3

Equation first becomes as

Ly(s)=2s+2-2s+3+e-s136s+16s2-14s+2+29s+3-e-5s1936s+16s2-74s+2+119s+3

Step 3: Take inverse Laplace transform we get

y(t)=2e-2t-2e-3t+136u(t-1)+16(t-1)u(t-1)-14e-2(t-1)u(t-1)+29e-3(t-1)u(t-1)-1936u(t-5)+16(t-5)u(t-5)-74e-2(t-5)u(t-5)+119e-3(t-5)u(t-5)=2e-2t-2e-3t+136+16(t-1)-14e-2(t-1)+29e-3(t-1)u(t-1)-1936+16(t-5)-74e-2(t-5)+119e-3(t-5)u(t-5)

Hence

y(t)=2e-2t-2e-3t+136+16(t-1)-14e-2(t-1)+29e-3(t-1)u(t-1)-1936+16(t-5)-74e-2(t-5)+119e-3(t-5)u(t-5)

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