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 Depdendent Variable

 Number of equations to solve: 23456789
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 Dependent Variable

 Number of inequalities to solve: 23456789
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 Solve for:

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# Linear Equations

L4 Linear Equations (Section 2.3)

Definition: A linear first order differential equation is an equation
that can be expressed in the form

Example:

Some Special Cases:

Discussion of the Method Involving an “Integrating Factor”:

1) Put the equation in the standard form:

where

2) Multiply both sides of the equation (4) by the integrating factor μ (x)

2) Determine μ (x) so that the left-hand side of the equation (5)
is the derivative of the product μ (x) y :

The general solution:

Method for Solving Linear Equations

(a) Write the equation in standard form

(b) Calculate the integrating factor μ (x) by the formula

(c) Give the general solution

where C is an arbitrary constant.

Example: Find the general solution to the equation

Example: Solve the initial value problem and find the value of y (−1).

Example: A rock contains two radioactive isotopes, RA1 and RA2 ,
that belong to the same radioactive series; that is, RA1 decays into
RA2 , which then decays into stable atoms. Assume that the rate at
which RA1 decays into RA2 is 20e−15t kg/sec. Because the rate of
decay of RA2 is proportional to the mass y (t ) of RA2 present, the
rate of change in RA2 is

[rate of change]= [rate of creation] – [rate of decay]

If k = 4 / sec and y (0) = 30 kg, find the mass y(t ) of RA2 for t ≥ 0

Existence and Uniqueness of Solution for a Linear First-order
Differential Equation

Theorem 1. Suppose P(x) and Q(x) are continuous on the
interval (a,b) that contains the point x0. Then for any choice of
initial value y0 , there exists a unique solution y (x) on (a,b) to the
initial value problem

In fact, the solution is

for a suitable value of C.

(See problem 34 for the details on the proof)

Special Cases when the Definite Integral is Used

Example: Solve the initial value problem

Example: Solve the initial value problem