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Fundamentals Of Differential Equations And Boundary Value Problems
Found in: Page 173
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

Using the mass-spring analogy, predict the behavior as t of the solution to the given initial value problem. Then confirm your prediction by actually solving the problem.

(a).y''+16y=0;y(0)=2,y'(0)=0(b).y''+100y'+y=0;y(0)=1,y'(0)=0(c).y''-6y'+8y=0;y(0)=1,y'(0)=0(d).y''+2y'-3y=0;y(0)=-2,y'(0)=0(e).y''-y'-6y=0;y(0)=1,y'(0)=1

  1. The solution is y=2cos4t.
  2. The general solution is y(t)=-50+249922499e(-50+2499)t+50+249922499e(-50-2499)t
  3. The general solution is y(t)=2e2te4t.
  4. The general solution is y(t)=-12e-3t-32et.
  5. The general solution is y(t)=-25e-3t+35et.
See the step by step solution

Step by Step Solution

Step 1: Find the general solution.

(a).

The differential equation is y''+16y=0.

The auxiliary equation is r2+16=0.

Find the roots of the auxiliary equation.

r2+16=0r=±4i

The general equation is y(t)=c1cos4t+c2sin4t.

Apply initial conditions y(0)=2,y'(0)=0.

Using the given initial values, we get:

y(t)=c1cos4t+c2sin4ty(0)=c1+0 2=c1y'(t)=-4c1sin4t+4c2cos4ty'(0)=-4c1sin0+4c2cos0 0=4c2 c2=0

Thus, the solution is y=2cos4t.

As 1cos4t1therefore the solution oscillates between -2 and 2.

Step 2: Check the result of the general solution

(b).

Here the differential equation is y''+100y'+y=0.

The auxiliary equation is r2+100r+1=0.

Find the roots of the auxiliary equation.

r2+100r+1=0r=-50±2499

The general equation is y(t)=c1e(-50+2499)t+c2e(-50-2499)t.

Apply initial conditions y(0)=1,y'(0)=0.

y(0)=c1=-50+249922499y'(0)=c2=50+249922499

The solution is y(t)=-50+249922499e(-50+2499)t+50+249922499e(-50-2499)t.

Since the powers of exponential functions tend to ast,y(t)0.

Step 3: Determine the solution

(c).

Here the differential equation is y''-6y'+8y=0.

The auxiliary equation is:

r2-6r+8=0r=2,4

The general equation is y(t)=c1e2t+c2e4t.

Apply initial conditions y(0)=1,y'(0)=0

y(0)=c1=2 y'(0)=c2=-1

The solution is y(t)=2e2te4t.

The solution approaches to   as  t.

Step 4: find the result.

(d).

Here the differential equation is y''+2y'-3y=0.

The auxiliary equation is:

r2+2r-3=0r=3,1

The general equation is y(t)=c1e-3t+c2et.

Apply initial conditions y(0)=-2,y'(0)=0

role="math" localid="1654848021100" y(0)=c1=-12y'(0)=c2=-32

The general solution is y(t)=-12e-3t-32et.

The solution approaches to   as  t.

Step 5: evaluate the result

(e).

Here the differential equation is y''-y'-6y=0.

The auxiliary equation is:

r2-r-6=0r=-2,3

The general equation is y(t)=c1e-2t+c2e3t.

Apply initial conditions y(0)=1,y'(0)=1

role="math" localid="1654848262559" y(0)=c1=-25y'(0)=c2=35

The solution is y(t)=-25e-3t+35et.

The solution approaches to   as  t.

This is the required result.

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