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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

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