The Computable Differential Equation Lecture ... - Bruce E. Shapiro
The Computable Differential Equation Lecture ... - Bruce E. Shapiro
The Computable Differential Equation Lecture ... - Bruce E. Shapiro
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CHAPTER 10. APPENDIX ON ANALYTIC METHODS<br />
(MATH 280 IN A NUTSHELL) 195<br />
Second Order Homogeneous Linear <strong>Equation</strong> with Constant Coefficients<br />
To solve the differential equation<br />
ay ′′ + by ′ + cy = 0<br />
find the roots of the characteristic equation<br />
ar 2 + br + c = 0<br />
If the roots (real or complex) are distinct, then<br />
If the roots are repeated then<br />
y = Ae r 1t + Be r 2t<br />
y = (A + Bt)e rt<br />
Method of Undetermined Coefficients<br />
To solve the differential equation<br />
ay ′′ + by ′ + cy = f(t)<br />
where f(t) is a polynomial, exponential, or trigonometric function, or any product<br />
thereof, the solution is<br />
y = y H + y P<br />
where y H is the complete solution of the homogeneous equation<br />
ay ′′ + by ′ + cy = 0<br />
To find y P make an educated guess based on the form form of f(t). <strong>The</strong> educated<br />
guess should be the product<br />
y P = P (t)S(t)E(t)<br />
where P (t) is a polynomial of the same order as in f(t). S(t) = r n (A sin rt+B cos rt)<br />
is present only if there are trig functions in rt in f(t), and n is the multiplicity of<br />
r as a root of the characteristic equation (n = 0 if r is not a root). E(t) = r n e rt<br />
is present only if there is an exponential in rt in f(t). If f(t) = f 1 (t) + f 2 (t) + · · ·<br />
then solve each of the equations<br />
ay ′′ + by ′ + cy = f i (t)<br />
separately and add all of the particular solutions together to get the complete particular<br />
solution.<br />
c○2007, B.E.<strong>Shapiro</strong><br />
Last revised: May 23, 2007<br />
Math 582B, Spring 2007<br />
California State University Northridge