I have this second-order ode equation: y''-4y'+13y=0Z

maduregimc

maduregimc

Answered question

2021-12-16

I have this second-order ode equation:
y4y+13y=0
I've identified it as x missing case as y=f(y,y)=4y13y, so I'm substituting witth:
y=P, y=Pdy2d2x=f(P,y)=4P13y
At this point I have Pdpdy=4P13y, which seems a non-linear first-order ODE. This is currently beyond the scope of my course, so I'm unsure if I should continue and search online for solving techniques, or did I already do something wrong?

Answer & Explanation

movingsupplyw1

movingsupplyw1

Beginner2021-12-17Added 30 answers

We have auxilliary equation:
m24m+13=0m=2±3i
Thus the general solution is:
y=e2x(Acos(3x)+Bsin(3x))
movingsupplyw1

movingsupplyw1

Beginner2021-12-18Added 30 answers

What do you mean as a x missing case ? This is a second order linear differential equation, not a first order one.
Assume a solution is proportional to eλx for some constant λ. Substitute y(x)=eλx into the ODE :
λ2eλx4λeλx+13eλx=0eλx(λ24λ+13)
=0λ=2±3i
The roots λ=2±3i give y1(x)=c1e(2+3i)x and y2(x)=e(23i)x. The general solution is the sum of the above equations :
y(x)=y1(x)+y2(x)yg(x)=c1e(2+3i)x+c2e(23i)x
Now, by applying Eulers
RizerMix

RizerMix

Expert2021-12-29Added 656 answers

In order to make your problem solvable in the manner in which you seek I think a substitution of y=e2tw is required. Here w is the new dependent variable. We calculate,
y=e2t(w+2w)
y=e2t(w+4w+4w)
substituting into t+4y+9y=0 yields:
e2t(w+9w)=0
hence solve the much easier problem w+9w=0 by a technique like the one you mention. Personally, I prefer the following trick w=vdvdw where v=dwdt
Thus, w+9w=0 yields
vdvdw+9w=0
or
vdv+9wdw=d(v2/2+9w2/2)=0
hence v2+9w2=9C2 is the solution. Solve for v=±9C29w2 But v=dwdt hence we reduce to the quadrature:
t=±dw(C29w2=±3|C|dw1w2/C2
Let u=w/C hence du=dw/C and
du1u2=sin1(u)+c1. Consequently,
t=±C3|C|(sin1(u)+c1)
Or,
sin1(w/C)=±3tc1
Yielding,
w=Csin(±3tc1)=Asin(3t+ϕ)
Since y=e2tw we conclude,
y=Ae2tsin(3t+ϕ)=c2e2tsin(3t)+c3e2tcos(3t)

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