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@@ -14,21 +14,21 @@ $$ ay'' + by' + cy = g(t) $$
This is a second-order differential equation with constant coefficients.
**Definition**. In the event that $g(t) = 0$, we say the equation is *homogenous*. Otherwise, the equation is *nonhomogenous*.
**Definition**. In the event that $g(t) = 0$, we say the equation is *homogenous*. Otherwise, the equation is *nonhomogeneous*.
**Definition**. Principal of Superposition. Let $y_1(t)$ and $y_2(t)$ be solutions to a linear, homogenous differential equation. Then, any linear combination of said solutions is also a solution to the differential equation. In other words, with $c_1, c_2 \in \mathbb{R}$, the following is a solution to a differential equation.
$$ y(t) = c_1 y_1(t) + c_2 y_2(t) $$
Given a second-order homogenous differential equation with constant coeffictions, we assume solutions of the following form:
Given a second-order homogenous differential equation with constant coefficients, we assume solutions of the following form:
$$ y(t) = e^{rt} $$
Substituting this equation into the differential equationm, we see the following:
Substituting this equation into the differential equation, we see the following:
$$ e^{rt}(ar^2 + br + c) = 0 $$
Thus, we allow the *charactaristic equation* of the differential equation to be as follows:
Thus, we allow the *characteristic equation* of the differential equation to be as follows:
$$ ar^2 + br + c = 0 $$
@@ -36,7 +36,7 @@ $$ ar^2 + br + c = 0 $$
This section is from [Paul's Online Math Notes](https://tutorial.math.lamar.edu/Classes/DE/RealRoots.aspx).
When the two roots to the charactaristic equation are discrete roots in the real numbers, we see the following solutions.
When the two roots to the characteristic equation are discrete roots in the real numbers, we see the following solutions.
$$ y_1(t) = e^{r_1 t} $$
@@ -50,7 +50,7 @@ $$ y(t) = c_1 e^{r_1 t} + c_2 e^{r_2 t} $$
This section is from [Paul's Online Math Notes](https://tutorial.math.lamar.edu/Classes/DE/ComplexRoots.aspx).
Let the solutions to the charactaristic equation be of the following form:
Let the solutions to the characteristic equation be of the following form:
$$ r_{1,2} = \lambda \pm \mu i $$
@@ -64,7 +64,7 @@ Recall Euler's Formula:
$$ e^{i \theta} = \cos \theta + i \sin \theta $$
A colliloquy of Euler's formula is the following:
A corollary of Euler's formula is the following:
$$ e^{-i \theta} = \cos(-\theta) + i \sin(-\theta) = \cos \theta - i \sin \theta $$
@@ -83,7 +83,7 @@ $$ y(t) = c_1 e^{\lambda t} \cos(\mu t) + c_2 e^{\lambda t} \sin(\mu t) $$
This section is from [Paul's Online Math Notes](https://tutorial.math.lamar.edu/Classes/DE/RepeatedRoots.aspx).
Assume the solutions to the charactaristic equations are $r = r_1 = r_2$. Thus, the two equations $y_t(t)$ and $y_2(t)$ are not linearly independent.
Assume the solutions to the characteristic equations are $r = r_1 = r_2$. Thus, the two equations $y_t(t)$ and $y_2(t)$ are not linearly independent.
After a *lot* of algebra, we see that
@@ -112,7 +112,7 @@ $$
**Definition**. If $W(f, g) \neq 0$, then $f(t)$ and $g(t)$ are said to form a *fundamental set of solutions*, and can be superimposed to form the general solution.
## Section 3.8 - Nonhomogenous Differential Equations
## Section 3.8 - Nonhomogeneous Differential Equations
This section is from [Paul's Online Math Notes](https://tutorial.math.lamar.edu/Classes/DE/NonhomogeneousDE.aspx).
@@ -120,11 +120,11 @@ Assume we have the differential equation as follows:
$$ y'' + p(t) y' + q(t) y = g(t) $$
The equivilent homogenous differential equation is
The equivalent homogenous differential equation is
$$ y'' + p(t) y' + q(t) y = 0 $$
**Theorem**. Assume $Y_1(t)$, $Y_2(t)$ are solutions to the nonhomogenous differential equations. Then, $Y_1(t) - Y_2(t)$ is a solution to the homogenous differential equation. This can be proved by substitution.
**Theorem**. Assume $Y_1(t)$, $Y_2(t)$ are solutions to the nonhomogeneous differential equations. Then, $Y_1(t) - Y_2(t)$ is a solution to the homogenous differential equation. This can be proved by substitution.
Thus, with $y_h(t)$ the solution to the homogenous problem, and $y_p(t)$ the solution to this particular problem, we can say that the general form of the solution to this differential equation is
@@ -156,7 +156,7 @@ Assume we have the differential equation as follows:
$$ y'' + p(t) y' + q(t) y = g(t) $$
The equivilent homogenous differential equation is
The equivalent homogenous differential equation is
$$ y'' + p(t) y' + q(t) y = 0 $$