19.6. Finding a Particular Integral. Introduction. Prerequisites. Learning Outcomes. Learning Style


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1 Finding a Particular Integral 19.6 Introduction We stated in Block 19.5 that the general solution of an inhomogeneous equation is the sum of the complementary function and a particular integral. We have seen how to find the complementary function in the case of a constant coefficient equation. We shall now deal with the problem of finding a particular integral. Recall that the particular integral is any solution of the inhomogeneous equation. There are a number of advanced techniques available for finding such solutions but we shall adopt a simpler strategy. Since any solution will do we shall try to find such a solution by a combination of educated guesswork and trial and error. Prerequisites Before starting this Block you should... Learning Outcomes After completing this Block you should be able to... understand what is meant by a particular integral find particular integrals by trial solution find general solutions of inhomogeneous equations by adding the complementary function to the particular integral 1 understand what is meant by a differential equation; (Block 19.1) 2 be familiar with the terminology associated with differential equations: order, dependent variable and independent variable; (Block 19.1) 3 be able to integrate; (Block 14) 4 have completed Block 19.5 on Constant Coefficient Equations Learning Style To achieve what is expected of you... allocate sufficient study time briefly revise the prerequisite material attempt every guided exercise and most of the other exercises
2 1. What is meant by a particular integral? Given a second order o.d.e. a d2 y dx + bdy + cy= f(x) 2 dx a particular integral is any function, y p (x), which satisfies the equation. That is, any function which when substituted into the left hand side and simplified, results in the function on the right. We denote a particular integral by y p (x). Try each part of this exercise Show that y = 1 4 e2x is a particular integral of Part (a) Starting with y = 1 4 e2x, find dy dx and d2 y dx 2 : Part (b) Now substitute these into (1): dx 2 dy dx 6y =e2x (1) Answer Answer 2. Finding a particular integral In the previous section we explained what is meant by a particular integral. Now we look at how one is actually found. In fact our method is rather crude. It involves trial and error and educated guesswork. We try solutions which are of the same general form as the f(x) onthe right hand side. As a guide, use Table 1. Table 1. Trial solutions to find the particular integral f(x) constant term c Trial solution constant term γ polynomial in x polynomial in x of degree r: of degree r: ax r + + bx + c αx r + + βx + γ a cos kx a sin kx ae kx α cos kx + β sin kx α cos kx + β sin kx αe kx Engineering Mathematics: Open Learning Unit Level 1 2
3 Example Find a particular integral of the equation dx 2 dy dx 6y =e2x (2) Solution We shall attempt to find a solution of the inhomogeneous problem by trying a function of the same form as that on the righthand side. In particular, let us try y(x) =αe 2x, where α is a constant that we shall now determine. If y(x) =αe 2x then Substitution in (2) gives: that is, dy dx =2αe2x and dx 2 =4αe2x. 4αe 2x 2αe 2x 6αe 2x =e 2x 4αe 2x =e 2x so that y will be a solution if α is chosen so that 4α = 1, that is, α = 1. Therefore the 4 particular integral is y p (x) = 1 4 e2x. Now do this exercise By trying a solution of the form y = αe x find a particular integral of the equation dx + dy 2y 2 =3e x dx Substitute y = αe x into the given equation to find α, and hence the particular integral. Answer Example Obtain a particular integral of the equation: dx 2 6dy dx +8y = x Solution In the last example, we found that a fruitful approach was to assume a solution in the same form as that on the righthand side. Suppose we assume a solution y(x) = αx and proceed to determine α. This approach will actually fail, but let us see why. If y(x) =αx then dy = α dx and d2 y = 0. Substitution into the differential equation yields 0 6α +8αx = x and α ought dx 2 now to be chosen so that this expression is true for all x. If we equate the coefficients of x we find 8α = 1 so that α = 1, but with this value of α the constant terms are inconsistent (that 8 is 6 on the left, but zero on the right). Clearly a particular integral of the form αx is not 8 possible. The problem arises because differentiation of the term αx produces constant terms which are unbalanced on the righthand side. So, we try a solution of the form y(x) =αx + β with α, β constants. This is consistent with the recommendation in Table 1. Proceeding as before dy = α, =0. dx dx 2 3 Engineering Mathematics: Open Learning Unit Level 1
4 Solution Substitution in the differential equation now gives: 0 6α +8(αx + β) =x Equating coefficients of x and then equating constant terms we find: 8α =1 ( ) 6α +8β =0 ( ) From ( ), α = 1 8 and then from ( ) 6 ( 1 8) +8β =0 so that, 8β = that is, β =. The required particular integral is 32 y p (x) = 1 8 x Try each part of this exercise Find a particular integral for the equation: dx 6dy +8y = 3 cos x 2 dx Part (a) First try to decide on an appropriate form for the trial solution. Refer to Table 1 if necessary Answer Part (b) Equate coefficients of cos x in your previous answer: Answer Part (c) Also, equate coefficients of sin x in your previous answer: Answer Part (d) Solve these simultaneously to find α and β, and hence the particular integral: Answer 3. Finding the general solution of a secondorder inhomogeneous equation The general solution of a secondorder linear inhomogeneous equation is the sum of its particular integral and the complementary function. In Block 19.5 you learned how to find a complementary function, and in the previous section you learnt how to find a particular integral. We now put these together to find the general solution. Engineering Mathematics: Open Learning Unit Level 1 4
5 Now do this exercise Find the general solution of dx +3dy 10y 2 =3x2 dx The complementary function was found in Block 19.5 page 6 to be y cf = Ae 2x + Be 5x. The particular integral is found by trying a solution of the form y = ax 2 + bx + c. Substitute into the homogeneous equation to find a, b and c, and hence y p (x). Answer Key Point The general solution of a constant coefficient ordinary differential equation a d2 y dx 2 + bdy dx + cy = f(x) is y = y p + y cf being the sum of the particular integral and the complementary function. y p contains no arbitrary constants; y cf contains two arbitrary constants. Example An LC circuit with sinusoidal input. The differential equation governing the flow of current in a series LC circuit when subject to an applied voltage v(t) = V 0 sin ωt is v L d2 i dt C i = ωv 0 cos ωt L C Obtain its general solution. i Solution The homogeneous equation is L d2 i cf dt 2 + i cf C =0. Letting i cf =e kt we find the auxiliary equation is Lk C = 0 so that k = ±i/ LC. Therefore, the complementary function is: i cf = A cos t t + B sin LC LC where A and B arbitrary constants 5 Engineering Mathematics: Open Learning Unit Level 1
6 Solution To find a particular integral try i p = E cos ωt + F sin ωt, where E, F are constants. We find: di p dt = ωe sin ωt + ωf cos ωt d 2 i p dt 2 Substitution into the inhomogeneous equation yields: = ω2 E cos ωt ω 2 F sin ωt L( ω 2 E cos ωt ω 2 F sin ωt)+ 1 C (E cos ωt + F sin ωt) =ωv 0 cos ωt Equating coefficients of sin ωt gives: ω 2 LF +(F/C)=0. Equating coefficients of cos ωt gives: ω 2 LE +(E/C)=ωV 0. Therefore F = 0 and E = CV 0 ω/(1 ω 2 LC). Hence the particular integral is Finally, the general solution is: i p = CV 0ω cos ωt. 1 ω 2 LC i = i cf + i p = A cos t LC + B sin t LC + CV 0ω cos ωt 1 ω 2 LC 4. Inhomogeneous term appearing in the complementary function Occasionally you will come across a differential equation a d2 y + b dy + cy = f(x) for which the dx 2 dx inhomogeneous term, f(x), forms part of the complementary function. One such example is the equation dx dy 6y 2 =e3x dx It is straightforward to check that the complementary function is y cf = Ae 3x + Be 2x. Note that the first of these terms has the same form as the inhomogeneous term, e 3x, on the righthand side of the differential equation. You should verify for yourself that trying a particular integral of the form y p (x) =αe 3x will not work in a case like this. Can you see why? Instead, try a particular integral of the form y p (x) =αxe 3x. Verify that dy p dx = αe3x (3x + 1) and p dx 2 = αe3x (9x +6). Substitute these expressions into the differential equation to find α = 1. Finally, the particular 5 integral is y p (x) = 1 5 xe3x and so the general solution to the differential equation is: y = Ae 3x + Be 2x xe3x Engineering Mathematics: Open Learning Unit Level 1 6
7 More exercises for you to try 1. Find the general solution of the following equations: (a) d2 x dt 2dx 3x =6. 2 dt (b) (d) d2 x dt +11dx +30x =8t 2 dt (e) (g) d2 y +9y =4e8x (h) dx2 d2 y dx 2 +5dy dx +4y = 8 d2 y dx +2dy +3y = 2 sin 2x 2 dx (f) d2 x 16x =9e6t dt2 2. Find a particular integral for the equation d2 x dt 3dx +2x 2 =5e3t dt 3. Find a particular integral for the equation d2 x dt 2 x =4e 2t 4. Obtain the general solution of y y 2y =6. (c) d2 y dt +5dy +6y =2t 2 dt d2 y dt + dy +y = 4 cos 3t 2 dt 5. Obtain the general solution of the equation dx +3dy +2y = 10 cos 2x. 2 dx dy Find the particular solution satisfying y(0)= 1, dx (0)=0. 6. Find a particular integral for the equation 7. Find the general solution of dx 2 + dy dx + y =1+x (a) d2 x dt 2 6dx dt +5x = 3 (b) d2 x dt 2 2dx dt + x =et Answer 7 Engineering Mathematics: Open Learning Unit Level 1
8 5. Computer Exercise or Activity For this exercise it will be necessary for you to access the computer package DERIVE. To solve a secondorder differential equation using DERIVE it is necessary to load what is called a Utility File named ode2. To do this is simple. Proceed as follows: In DERIVE, choose File:Load:Math and select the file (double click) on the ode2 icon. This will load a number of commands which enable you to solve secondorder differential equations. You can use the Help facility to learn more about these if you wish. Of particular relevance here is the command Dsolve2(p, q, r, x, c1,c2) which finds the general solution (containing two arbitrary constants c1,c2) to the second order differential equation dx + p(x)dy + q(x)y = r(x) 2 dx For the examples in this Block both p(x) and q(x) are given constants. The general solution is the sum of the complementary function (the part containing the arbitrary constants) and the particular integral. Hence by inspecting the solution given by DERIVE the particular integral can be obtained. For example the general solution to can be obtained by keying Author:Expression dx dy 6y 2 =e2x dx Dsolve2( 1, 6, exp(2x), c1, c2) followed by Simplify and DERIVE responds with c1 ê 3 x ê2 x from which we deduce the particular integral: 4 + c2 ê 2 x ê2 x 4 As an exercise use DERIVE to check the correctness of the particular integrals requested in the examples and guided exercises of this Block. Engineering Mathematics: Open Learning Unit Level 1 8
9 MAPLE will solve a wide range of ordinary differential equations including systems of differential equations using the command dsolve(deqns,vars,eqns) where: deqns ordinary differential equation in vars, or set of equations and/or initial conditions. vars variable or set of variables to be solved for eqns optional equation of the form keyword=value For example to solve dt +2dy 2 dt +2y =e t y(0)=0,y (0)=0 we would key in > dsolve({diff(y(t),t$2)+2*diff(y(t),t)+2*y(t)=exp(t),y(0)=0, D(y)(0)=0},y(t),type=exact); MAPLE responds with 1 cos(t) e t If the initial conditions are omitted MAPLE will present the solution with the correct number of arbitrary constants denoted by C1, C2... Thus the general solution of dt +2dy +2y 2 =e t dt is obtained by keying in > dsolve({diff(y(t),t$2)+2*diff(y(t),t)+2*y(t)=exp(t)},y(t),type=exact); and MAPLE responds with y(t) = exp( t)+ C1 exp( t) cos(t)+ C2 exp( t) sin(t) As with the DERIVE response the particular integral can be deduced from this general solution as being that part not multiplied by an arbitrary constant. 9 Engineering Mathematics: Open Learning Unit Level 1
10 End of Block 19.6 Engineering Mathematics: Open Learning Unit Level 1 10
11 dy dx = 1 2 e2x, dx 2 = e 2x Back to the theory 11 Engineering Mathematics: Open Learning Unit Level 1
12 Substitution into (1) yields e 2x ( 1 2 e2x) 6 ( 1 4 e2x) which simplifies to e 2x, the same as the right hand side. Therefore y = 1 4 e2x is a particular integral and we write (attaching a subscript p) y p (x) = 1 4 e2x Back to the theory Engineering Mathematics: Open Learning Unit Level 1 12
13 α = 3 2 ; y p(x) = 3 2 e x Back to the theory 13 Engineering Mathematics: Open Learning Unit Level 1
14 y = α cos x + β sin x in which α, β are constants to be found. We shall try a solution of the form y(x) =α cos x + β sin x. Differentiating, we find: dy d = α sin x + β cos x dx Substitution into the differential equation gives: 2 y = α cos x β sin x dx2 ( α cos x β sin x) 6( α sin x + β cos x)+8(α cos x + β sin x) = 3 cos x Back to the theory Engineering Mathematics: Open Learning Unit Level 1 14
15 7α 6β =3 Back to the theory 15 Engineering Mathematics: Open Learning Unit Level 1
16 7β +6α =0 Back to the theory Engineering Mathematics: Open Learning Unit Level 1 16
17 α = 21 18,β=,y p(x) = cos x sin x Back to the theory 17 Engineering Mathematics: Open Learning Unit Level 1
18 a = 3, b = 9, c = 57, y p(x) = 3 10 x2 9 x 57. Thus the general solution is y = y p (x)+y cf (x) = 3 10 x x Ae2x + Be 5x Back to the theory Engineering Mathematics: Open Learning Unit Level 1 18
19 1. (a) x = Ae t + Be 3t 2 (b) y = Ae x + Be 4x + 2 (c) y = Ae 2t + Be 3t + 1t (d) x = Ae 6t + Be 5t t (e) y =e x [A sin 2x + B cos 2x] 8 2 cos 2x sin 2x (f) y =e 0.5t (A cos 0.866t + B sin 0.866t) cos 3t sin 3t (g) y = A cos 3x + B sin 3x e 8x (h) x = Ae 4t + Be 4t e6t 2. x p =2.5e 3t 3. x p = 4 3 e 2t 4. y = Ae 2x + Be x 3 5. y = Ae 2x + Be x + 3 sin 2x 1 cos 2x, e 2x + 3 sin 2x 1 cos 2x y p = x 7. (a) x = Ae t + Be 5t + 3 (b) x = Ae t + Bte t t2 e t Back to the theory 19 Engineering Mathematics: Open Learning Unit Level 1
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