# A5 - Applied Math 250 Assignment#5 Spring 2009 due June...

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Unformatted text preview: Applied Math 250 Assignment #5 Spring 2009 due June 16th A/ Problem Set 3 #1, #2, #3, #8 B/ Other recommended problems from Problem Set 3 (not for submission): #4, #6, #7 I/ 11/ 111/ Solve the IVP y” + y’ — 6y = 26-3“, y(0) = 0, y’(0) = 1. The technique we’ve developed works for third-order equations as well (as long as they’re linear, with constant coefﬁcients). The only complications are that the characteristic polynomial will be of third degree, and we require three linearly independent solutions, so that we can accommodate three initial conditions. Find the general solutions to the following equations (hint: if you haven’t had experi- ence in factoring cubics, note that (:1: — a)(m — b)(:r — c) = m3 + — abc, so if we can factor the constant term we should be able to identify a real root of a cubic by inspection, and then use long division to find the remaining quadratic factor). a) ym _ 3y” + 3y! _ y = e—t b) y’” + 3y”+12y’ +10y = 0 Unfortunately, if we encounter a 2nd-order equation which is either nonlinear or linear with nonconstant coefficients, we have little hope of solving it. Here are a few exceptions: o If y does not appear in the equation explicitly, then we can view the equation as a 1st-order equation for y’, and then integrate to ﬁnd y. This can be formalized by making the substitution u = 31’, so that u’ = 3/”. Find the general solutions to the following problems: 1 _ a)y//+;y1_0 b) y”+2my’=0 Note: integrating y’ is a problem in The best we can do is to express the result in terms of the “error function”: erf(:r) = if f0: 6"2 dt. o If x does not appear in the equation explicitly, it turns out that the same dy dgy du du dy du ‘bst't t‘nma hel. Letu=—,so——=-——=——=u—, and b“ 1 u 10 y p dm doc? d2: dy dz dy then we can express the DE as a 1st-order DE for 11(y). Unfortunately, this frequently proves fruitless anyway, since we may not be able to solve the resulting lst-order DES! Convert y” + y’ + y2 = 0 into a lst-order DE for u = 3%. This will be neither linear nor separable, so you won’t be able to solve it. C) d) Convert y” = y2 into a lst-order DE for u, and solve it, to obtain a lst—order DE for We get further this time, but we still can’t ﬁnish the problem. 6) Now try y” = 2yy’. You should ﬁnd that either y = C (a constant), or i3 y2 + K. Proceeding from this point requires three cases, for K > O, K < 0, and K = 0; including the equilibrium solutions y = C there are AM 250 ~ Assignment #5 Page 2 actually 4 completely different families of solutions! (There is no guarantee of uniqueness for nonlinear second—order initial value problems.) Assume K = O and ﬁnd one of these families. Verify that your result satisﬁes the DE. ...
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A5 - Applied Math 250 Assignment#5 Spring 2009 due June...

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