Math Problem Statement

Solve y'' + 4y' - 5y = 0, y(0) = 3, y'(0) = -3

Solution

This problem involves solving a second-order linear homogeneous differential equation with initial conditions. The equation is:

y+4y5y=0,y(0)=3,y(0)=3.y'' + 4y' - 5y = 0, \quad y(0) = 3, \quad y'(0) = -3.

Step 1: Finding the General Solution

  1. Form the characteristic equation for the differential equation:

r2+4r5=0.r^2 + 4r - 5 = 0.

  1. Solve the characteristic equation using the quadratic formula r=b±b24ac2ar = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}, where a=1a = 1, b=4b = 4, and c=5c = -5:

r=4±4241(5)21=4±16+202=4±362=4±62.r = \frac{-4 \pm \sqrt{4^2 - 4 \cdot 1 \cdot (-5)}}{2 \cdot 1} = \frac{-4 \pm \sqrt{16 + 20}}{2} = \frac{-4 \pm \sqrt{36}}{2} = \frac{-4 \pm 6}{2}.

This gives two distinct roots:

r1=1,r2=5.r_1 = 1, \quad r_2 = -5.

  1. General solution for the differential equation:

y(t)=C1er1t+C2er2t=C1et+C2e5t.y(t) = C_1 e^{r_1 t} + C_2 e^{r_2 t} = C_1 e^t + C_2 e^{-5t}.

Step 2: Applying Initial Conditions

  1. Initial condition 1: y(0)=3y(0) = 3.

y(0)=C1e0+C2e50=C1+C2=3.y(0) = C_1 e^0 + C_2 e^{-5 \cdot 0} = C_1 + C_2 = 3.

  1. Initial condition 2: y(0)=3y'(0) = -3.

First, compute the derivative of y(t)y(t):

y(t)=C1et5C2e5t.y'(t) = C_1 e^t - 5C_2 e^{-5t}.

Now, apply the initial condition:

y(0)=C1e05C2e50=C15C2=3.y'(0) = C_1 e^0 - 5C_2 e^{-5 \cdot 0} = C_1 - 5C_2 = -3.

Step 3: Solving the System of Equations

We now have the following system of equations:

  1. C1+C2=3C_1 + C_2 = 3.
  2. C15C2=3C_1 - 5C_2 = -3.

Solve these equations simultaneously:

From equation 1: C1=3C2C_1 = 3 - C_2.

Substitute C1C_1 into equation 2:

(3C2)5C2=336C2=36C2=6C2=1.(3 - C_2) - 5C_2 = -3 \Rightarrow 3 - 6C_2 = -3 \Rightarrow -6C_2 = -6 \Rightarrow C_2 = 1.

Now, find C1C_1:

C1=3C2=31=2.C_1 = 3 - C_2 = 3 - 1 = 2.

Step 4: Final Solution

The solution to the differential equation is:

y(t)=2et+e5t.y(t) = 2e^t + e^{-5t}.

Would you like further details or have any questions on this solution?


Related Questions:

  1. How would the solution change if the coefficients in the differential equation were different?
  2. What happens if the initial conditions are altered?
  3. How can we verify the solution using software tools?
  4. How do we handle non-homogeneous differential equations?
  5. What if the characteristic equation has complex roots?

Tip:

When solving homogeneous linear differential equations, always start by finding the characteristic equation and solving for its roots to construct the general solution.

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Linear Homogeneous Differential Equations
Initial Conditions

Formulas

Characteristic equation: r^2 + 4r - 5 = 0
Quadratic formula: r = (-b ± √(b^2 - 4ac)) / 2a
General solution: y(t) = C1e^(r1t) + C2e^(r2t)

Theorems

Quadratic Formula
General Solution of Linear Homogeneous Equations

Suitable Grade Level

University level or advanced high school