How to solve for the equation y′′′+4y′′+5y′+2y=10cost using laplace transform

ghostmasakii

ghostmasakii

Answered question

2022-01-28

How to solve for the equation y′′′+4y′′+5y′+2y=10cost using laplace transform method given that y(0)=0,y′(0)=0,and y′′(0)=3

Answer & Explanation

alenahelenash

alenahelenash

Expert2022-03-10Added 556 answers

Given differential equation 

y+4y+5y+2y=10cost

First we will find the complementary part of the solution that is solution of the homogeneous part

y+4y+5y+2y=0

The auxiliary equation is 
m2+4m2+5m+2=0m3+2m2+2m2+4m+m+2=0m2(m+2)+2m(m+2)+1(m+2)=0(m+2)(m2+2m+1)=0(m+2)(m+1)2=0m=2,1,1

So the complementary part of the solution yc(t)=ae2t+bet+ctet where a,b,c are arbitrary constant.

By method of undetermined coefficients suppose the particular solution is ,

yp(t)=Acos(t)+Bsin(t)

Then,

yp(t)=Asin(t)+Bcos(t)yp(t)=Acos(t)+Bsin(t)yp(t)=Asin(t)Bcos(t)

Substituting these in the given differential equation we get,

(asin(t)Bcos(t))+4(Acos(t)Bsin(t))+5(Asin(t)+Bcos(t))+2(Acos(t)+Bsin(t))=10cos(t)

(4A2B)sint+(2A+4B)cost=10cos(t)

Equating coefficients of sint,cost both side we get ,

4A2B=0 and 2A+4B=10

Solving these we have A=1 and B=2

So the particular solution is yp(t)=cos(t)+2sin(t)

So the general solution is ,

y(t)=ae2t+bet+ctetcos(t)+2sin(t)

Now y(0)=0 gives,

a+b1=0.....(i)

y(0)=0 gives,

2ab+c+2=0.....(ii)

y(0)=0 gives,

4a+b2c+1=0.....(iii)

Solving (i), (ii) and (iii) we get,

a=4,b=5 and c=5

Hence the required solution is ,

y(t)=4e2t+5et5tetcos(t)+2sin(t)

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