Question

Solve the given initial value problem. Sketch the graph of the solution and describe its behavior for increasing \(t.\) \(9 y^{\prime \prime}+6 y^{\prime}+82 y=0, \quad y(0)=-1, \quad y^{\prime}(0)=2\)

Short answer

Short Answer: The particular solution of the given initial value problem is \(y(t)=e^{-\frac{1}{3}t}[-\cos(\frac{\sqrt{696}}{18}t)+\frac{9}{2}\sin(\frac{\sqrt{696}}{18}t)]\). Its graph represents damped oscillations, where the amplitude of the oscillations decreases as time goes on, eventually approaching zero. The solution indicates a stable equilibrium as time increases.

Step-by-step solution

Identify the given differential equation and initial conditions

The given differential equation is \(9y^{\prime\prime}+6y^{\prime}+82y=0\) with initial conditions \(y(0)=-1\) and \(y^{\prime}(0)=2\).

Determine the characteristic equation

The characteristic equation is found by replacing \(y^{\prime\prime}\) with \(r^2,\) \(y^{\prime}\) with \(r,\) and \(y\) with \(1\). This gives the characteristic equation: \(9r^2+6r+82=0\).

Solve the characteristic equation

To solve the quadratic equation \(9r^2+6r+82=0\), we use the quadratic formula: \(r=\frac{-b\pm\sqrt{b^2-4ac}}{2a}\). In this case, \(a=9\), \(b=6\), and \(c=82\). So, the roots are: \(r=\frac{-6\pm\sqrt{6^2-4(9)(82)}}{2(9)}\). Since the discriminant \(b^2-4ac\) is negative, we'll have complex roots. Therefore, the general solution of the differential equation will be in the form: \(y(t)=e^{\alpha t}[A\cos(\beta t)+B\sin(\beta t)]\), where \(\alpha\) is the real part of the roots and \(\beta\) is the imaginary part of the roots.

Find the general solution of the differential equation

We obtain the real part \(\alpha\) and the imaginary part \(\beta\) from our roots: \(\alpha=\frac{-6}{2(9)}=-\frac{1}{3}\), and \(\beta=\frac{\sqrt{|6^2-4(9)(82)|}}{2(9)}=\frac{\sqrt{696}}{18}\). Now, we substitute these values into the general solution form: \(y(t)=e^{-\frac{1}{3}t}[A\cos(\frac{\sqrt{696}}{18}t)+B\sin(\frac{\sqrt{696}}{18}t)]\).

Apply the initial conditions to obtain the particular solution

Apply the initial condition \(y(0)=-1\): \(-1=e^{0}[A\cos(0)+B\sin(0)]\) which simplifies to \(A=-1\). Apply the initial condition \(y^{\prime}(0)=2\): First, find the derivative \(y^{\prime}(t)\): \(y^{\prime}(t)=-\frac{1}{3}e^{-\frac{1}{3}t}[-A\cos(\frac{\sqrt{696}}{18}t)-B\sin(\frac{\sqrt{696}}{18}t)]+\frac{\sqrt{696}}{18}e^{-\frac{1}{3}t}[-A\sin(\frac{\sqrt{696}}{18}t)+B\cos(\frac{\sqrt{696}}{18}t)]\) Substitute \(A=-1\) and evaluate at \(t=0\): \(2=-\frac{1}{3}e^{0}[-(-1)\cos(0)-B\sin(0)]+\frac{\sqrt{696}}{18}e^{0}[-(-1)\sin(0)+B\cos(0)]\) which simplifies to \(B=\frac{9}{2}\). The particular solution is: \(y(t)=e^{-\frac{1}{3}t}[-\cos(\frac{\sqrt{696}}{18}t)+\frac{9}{2}\sin(\frac{\sqrt{696}}{18}t)]\).

Sketch the graph of the particular solution and describe its behavior for increasing \(t\)

The graph of particular solution represents a damped oscillation, due to the presence of the exponential term \(e^{-\frac{1}{3}t}\) and the trigonometric functions. As \(t\) increases, the amplitude of the oscillations will decrease, eventually approaching zero. This means that the solution approaches a stable equilibrium as time goes on.

Key concepts

Initial Value Problems

Initial value problems are a common type of differential equation problem where you're asked to find a specific solution that satisfies given conditions. These conditions, called "initial conditions," help determine the unique solution out of many possible solutions for a differential equation.
In the given exercise, we're working with a second-order linear homogeneous differential equation: \(9y^{\prime\prime}+6y^{\prime}+82y=0\). The associated initial conditions are \(y(0)=-1\) and \(y^{\prime}(0)=2\).
The goal is to find a particular solution \(y(t)\) that not only satisfies the differential equation but also meets these specified initial conditions. Establishing initial conditions is crucial because a second-order differential equation typically has a family of solutions. The initial conditions narrow it down to one specific solution that behaves in a specified manner at the start (\(t=0\)).

Characteristic Equation

The characteristic equation plays a key role in solving linear differential equations, especially with constant coefficients. To find it, we substitute elements from the differential equation into a polynomial equation. In this case, we have \(9r^2+6r+82=0\).
This characteristic equation is derived by substituting \(y^{\prime\prime}\) with \(r^2\), \(y^{\prime}\) with \(r\), and \(y\) itself with \(1\), forming a quadratic equation for \(r\), the "characteristic root."
Solving the characteristic equation helps us determine the nature of the roots, which indicates whether the solution involves real numbers, or in this case, complex numbers. This information is important for determining the type of solution you'll construct for the differential equation.

Complex Roots

When solving a quadratic characteristic equation, like \(9r^2+6r+82=0\), you check the discriminant ( \(b^2-4ac\)) to determine the types of roots. Here, the discriminant is negative, indicating complex roots.
Complex roots arise in conjugate pairs and are expressed as \(\alpha \pm i\beta\). In the general solution of the differential equation with complex roots, you'll use an exponential function combined with sine and cosine functions: \[y(t)=e^{\alpha t}\big[A\cos(\beta t)+B\sin(\beta t)\big]\] In this equation, \(\alpha\) is the real component, which affects the damping effect, and \(\beta\) is the imaginary component, which controls the oscillation.
In our solution, \(\alpha = -\frac{1}{3}\) and \(\beta = \frac{\sqrt{696}}{18}\). These values guide the construction of the "shape" of our solution as it evolves over time.

Damped Oscillations

Damped oscillations describe a phenomenon where oscillations decrease in amplitude over time, often due to a resistance or damping force. They are common in systems where energy is lost, like a swinging pendulum that eventually comes to a stop.
In our differential equation solution, the term \(e^{\alpha t}\) (where \(\alpha\) is negative) causes the amplitude to decay exponentially as time increases, leading to damped oscillations.
The presence of sine and cosine functions, such as \(-\cos\) and \(\sin\) in our particular solution, indicate oscillating behavior. The combination of these trigonometric functions with the exponential decay reflects how the solution behaves as a damped oscillator.
As time \(t\) grows, the overall impact of \(e^{-\frac{1}{3}t}\) causes the oscillations to gradually diminish, leading them toward zero. This results in the solution approaching a stable equilibrium over time.