# SECOND ORDER (inhomogeneous)

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1 Differential Equations SECOND ORDER (inhomogeneous) Graham S McDonald A Tutorial Module for learning to solve 2nd order (inhomogeneous) differential equations Table of contents Begin Tutorial c 2004

2 1. Theory 2. Exercises 3. Answers 4. Standard derivatives 5. Finding y CF 6. Tips on using solutions Full worked solutions Table of contents

3 Section 1: Theory 3 1 Theory This Tutorial deals with the solution of second order linear o.d.e. s with constant coefficients (a, b and c), i.e. of the form: a d2 y dx 2 + b dy + cy = f(x) dx ( ) The first step is to find the general solution of the homogeneous equation [i.e. as ( ), except that f(x) = 0]. This gives us the complementary function y CF. The second step is to find a particular solution y P S of the full equation ( ). Assume that y P S is a more general form of f(x), having undetermined coefficients, as shown in the following table:

4 Section 1: Theory 4 f(x) k (a constant) linear in x quadratic in x k sin px or k cos px ke px sum of the above product of the above (where p is a constant) Form of y P S C Cx + D Cx 2 + Dx + E C cos px + D sin px Ce px sum of the above product of the above Note: If the suggested form of y P S already appears in the complementary function then multiply this suggested form by x. Substitution of y P S into ( ) yields values for the undetermined coefficients (C, D, etc). Then, General solution of ( ) = y CF + y P S.

5 Section 2: Exercises 5 2 Exercises Find the general solution of the following equations. Where boundary conditions are also given, derive the appropriate particular solution. Click on Exercise links for full worked solutions (there are 13 exercises in total) [ Notation: y = d2 y dx 2, y = dy ] dx Exercise 1. y 2y 3y = 6 Exercise 2. y + 5y + 6y = 2x Exercise 3. (a) y + 5y 9y = x 2 (b) y + 5y 9y = cos 2x (c) y + 5y 9y = e 4x (d) y + 5y 9y = e 2x + 2 x Exercise 4. y λ 2 y = sin 2x Theory Answers Derivatives Finding y CF Tips

6 Section 2: Exercises 6 Exercise 5. y y = e x Exercise 6. y + y 2y = e 2x Exercise 7. y 2y + y = e x Exercise 8. y + 8y + 17y = 2e 3x ; y(0) = 2 and ( π ) y = 0 2 Exercise 9. y + y 12y = 4e 2x ; y(0) = 7 and y (0) = 0 Exercise 10. y + 3y + 2y = 10 cos 2x ; y(0) = 1 and y (0) = 0 Exercise 11. y + 4y + 5y = 2e 2x ; y(0) = 1 and y (0) = 2 Exercise 12. d2 x dτ 2 +4dx dτ +3x = e 3τ ; Exercise 13. d2 y dτ dy + 5y = 6 sin τ dτ x= 1 2 dx and = 2 at τ = 0 dτ Theory Answers Derivatives Finding y CF Tips

7 Section 3: Answers 7 3 Answers 1. y = Ae x + Be 3x 2, 2. y = Ae 2x + Be 3x + x , 3. General solutions are y = y CF + y P S where y CF = Ae m1x + Be m2x ( m1,2 = 5 2 ± 1 2 and (a) y P S = 1 9 x x (b) y P S = cos 2x sin 2x (c) y P S = 1 27 e4x (d) y P S = 1 15 e 2x x 13 81, 4. y = Ae +λx + Be λx 5. y = Ae x + Be x xex, sin 2x 4+λ 2, 61 )

8 Section 3: Answers 8 6. y = Ae x + Be 2x 1 3 xe 2x, 7. y = (A + Bx)e x x2 e x, 8. y = e 4x (A cos x + B sin x) + e 3x ; y = e 4x (cos x e π 2 sin x) + e 3x, 9. y = Ae 3x + Be 4x 2 3 e2x ; y = 32 7 e3x e 4x 2 3 e2x, 10. y = Ae 2x + Be x 1 2 cos 2x sin 2x ; y = 3 2 e 2x 1 2 cos 2x sin 2x, 11. y = e 2x (A cos x + B sin x) + 2e 2x ; y = e 2x (2 cos x), 12. x = Ae 3τ + Be τ 1 2 τe 3τ ; x = 1 2 (1 τ)e 3τ, 13. y = e 2τ (A cos τ + B sin τ) 3 4 (cos τ sin τ).

9 Section 4: Standard derivatives 9 4 Standard derivatives f(x) f (x) f(x) f (x) x n n x n 1 [g(x)] n n[g(x)] n 1 g (x) ln x 1 x (x > 0) ln g(x) 1 g(x) g (x) (g(x) > 0) e x e x a x a x ln a (a > 0) sin x cos x sinh x cosh x cos x sin x cosh x sinh x tan x sec 2 x tanh x sech 2 x cosec x cosec x cot x cosech x cosech x coth x sec x sec x tan x sech x sech x tanh x cot x cosec 2 x coth x cosech 2 x

10 Section 4: Standard derivatives 10 f(x) f (x) f(x) f (x) tan 1 x 1 1+x 2 tanh 1 x 1 1 x 2 ( 1 < x < 1) cos 1 x 1 ( 1 < x < 1) 1 x 2 cosh 1 1 x x (x > 1) 2 1 sin 1 x 1 1 x 2 ( 1 < x < 1) sinh 1 x 1 x 2 +1 where cosec x = 1/ sin x, sec x = 1/ cos x, cot x = 1/ tan x cosech x = 1/ sinh x, sech x = 1/ cosh x, coth x = 1/ tanh x

11 Section 5: Finding y CF 11 5 Finding y CF One considers the differential equation with RHS = 0. Substituting a trial solution of the form y = Ae mx yields an auxiliary equation : am 2 + bm + c = 0. This will have two roots (m 1 and m 2 ). The general solution y CF, when RHS = 0, is then constructed from the possible forms (y 1 and y 2 ) of the trial solution. The auxiliary equation may have: i) real different roots, m 1 and m 2 y CF = y 1 + y 2 = Ae m1x + Be m2x or ii) real equal roots, m 1 = m 2 y CF = y 1 + xy 2 = (A + Bx)e m1x or iii) complex roots, p ± iq y CF = y 1 + y 2 e px (A cos qx + B sin qx)

12 Section 6: Tips on using solutions 12 6 Tips on using solutions When looking at the THEORY, ANSWERS, DERIVATIVES, FIND- ING y CF or TIPS pages, use the Back button (at the bottom of the page) to return to the exercises Use the solutions intelligently. For example, they can help you get started on an exercise, or they can allow you to check whether your intermediate results are correct Try to make less use of the full solutions as you work your way through the Tutorial

13 Solutions to exercises 13 Full worked solutions Exercise 1. y 2y 3y = 6 Auxiliary equation (A.E.) from the homogeneous equation is m 2 2m 3 = 0 y 2y 3y = 0, i.e. (m 3)(m + 1) = 0 i.e. m 1 = 3, m 2 = 1. Real different roots : homogeneous equation has general solution y = Ae m1x + Be m2x i.e. y CF = Ae x + Be 3x (complementary function).

14 Solutions to exercises 14 f(x) = 6 y P S = C y P S = y P S = 0 suggests form of particular solution (C = undetermined constant) when y P S = constant substitute y P S = C : C = 6 i.e. C = 2. General solution, y = y CF + y P S = Ae x + Be 3x 2. Return to Exercise 1

15 Solutions to exercises 15 Exercise 2. y + 5y + 6y = 2x A.E. is m 2 + 5m + 6 = 0 i.e. (m + 2)(m + 3) = 0 i.e. m 1 = 2, m 2 = 3, y CF = Ae 2x + Be 3x. f(x) = 2x suggests substitution of y P S = Cx + D (C, D are undetermined constants) y P S = C, y P S = 0, substitution gives 0 + 5C + 6(Cx + D) = 2x i.e. 5C + 6D + 6Cx = 2x Constant term : 5C + 6D = 0 Coefficient of x: 6C = 2 i.e. C = 1 3 and D = 5C 6 = 5 18

16 Solutions to exercises 16 General solution, y = y CF + y P S i.e. y = Ae 2x + Be 3x x Return to Exercise 2

17 Solutions to exercises 17 Exercise 3. Parts (a)-(d) have same homogeneous equation i.e. y + 5y 9y = 0 with A.E. m 2 + 5m 9 = 0 ( i.e. m = ± ) 25 ( 36) i.e. m = 5 2 ± Real different roots: y CF = Ae m1x + Be m2x, (a) y + 5y 9y = x 2 where m 1 = m 2 = Try y P S = Cx 2 + Dx + E, dy P S dx = 2Cx + D, d 2 y P S dx 2 = 2C

18 Solutions to exercises 18 Substitution: 2C + 5(2Cx + D) 9(Cx 2 + Dx + E) = x 2 i.e. 2C + 5D 9E = 0 (constant term) 10Cx 9Dx = 0 (terms in x) 9Cx 2 = x 2 (terms in x 2 ) 9C = 1 i.e. C = C 9D = 0 gives D = 10C 9 = C + 5D 9E = 0 gives E = 1 9 (2C + 5D) ( = General solution is: ) = 1 9 ( ) = y = y CF + y P S = Ae m1x + Be m2x 1 9 x x

19 Solutions to exercises 19 (b) y + 5y 9y = cos 2x Try y P S = C cos 2x + D sin 2x y P S = 2C sin 2x + 2D cos 2x y P S = 4C cos 2x 4D sin 2x Substitute: 4C cos 2x 4D sin 2x + 5 ( 2C sin 2x + 2D cos 2x) 9(C cos 2x + D sin 2x) = cos 2x i.e. i.e. ( 4C +10D 9C) cos 2x (4D+10C +9D) sin 2x = cos 2x ( 13C + 10D) cos 2x (13D + 10C) sin 2x = cos 2x Coefficients of cos 2x: 13C + 10D = 1 (i) Coefficients of sin 2x: 13D + 10C = 0 (ii)

20 Solutions to exercises 20 (ii) gives D = 10C 13 then (i) gives 13C 100C 13 = 1 i.e = 1 C i.e = C General solution is: ( ) then D = = i.e. y P S = cos 2x sin 2x y = y CF + y P S = Ae m1x + Be m2x + 1 (10 sin 2x 13 cos 2x). 269

21 Solutions to exercises 21 (c) Try i.e. y + 5y 9y = e 4x y P S = Ce 4x y P S = 4Ce4x y P S = 16Ce4x Substitute: i.e. 16Ce 4x + 20Ce 4x 9Ce 4x = e 4x (16C + 20C 9C)e 4x = e 4x i.e. 27C = 1 i.e. C = 1 27 (by comparing coefficients of e 4x ) i.e. y P S = 1 27 e4x General solution is: y = y CF + y P S = Ae m1x + Be m2x e4x.

22 Solutions to exercises 22 (d) y + 5y 9y = e 2x + 2 x Try y P S = Ce 2x +Dx+E (i.e. sum of the forms ke px and linear in x ) i.e. y P S = 2Ce 2x +D y P S = +4Ce 2x Substitute: 4Ce 2x 10Ce 2x +5D 9Ce 2x 9Dx 9E =e 2x +2 x Coeff. e 2x : 4C 10C 9C = 1 C = 1 15 Coeff. x : 9D = 1 D = 1 9 Coeff. x 0 : 5D 9E = 2 5 ( 1 9) 9E = 2 i.e. y P S = 1 15 e 2x x i.e. E = 13 81

23 Solutions to exercises 23 General solution is: y = y CF + y P S = Ae m1x + Be m2x 1 15 e 2x x Return to Exercise 3

24 Solutions to exercises 24 Exercise 4. y λ 2 y = sin 2x A.E. is m 2 λ 2 = 0 i.e. m = ±λ (real different roots) y CF = Ae λx + Be λx f(x) = sin 2x suggests trying Substitute: i.e. y P S = A sin 2x + B cos 2x y P S = 2A cos 2x 2B sin 2x y P S = 4A sin 2x 4B cos 2x 4A sin 2x 4B cos 2x λ 2 A sin 2x λ 2 B cos 2x = sin 2x i.e. ( 4A λ 2 A) sin 2x + ( 4B λ 2 B) cos 2x = sin 2x Coeff. cos 2x : 4B λ 2 B = 0 i.e. B = 0 Coeff. sin 2x : 4A λ 2 A = 1 i.e. A = 1 4+λ 2 sin 2x i.e. y P S = 4+λ ; 2 general solution y = y CF + y P S = Ae λx + Be λx sin 2x 4+λ 2. Return to Exercise 4

25 Solutions to exercises 25 Exercise 5. y y = e x A.E. is m 2 1 = 0 i.e. m = ±1, y CF = Ae x + Be x f(x) = e x suggests trying y P S = Ce x BUT THIS ALREADY AP- PEARS IN y CF, therefore multiply this trial form by x. Try Substitute: y P S = Cxe x, y P S = Cxex + Ce x = C(1 + x)e x y P S = C(1 + x)ex + Ce x = C(2 + x)e x C(2 + x)e x Cxe x = e x Coeff. e x : C(2 + x) Cx = 1 i.e. 2C = 1 i.e. C = 1 2 y P S = 1 2 xex and general solution is y = y CF + y P S = Ae x + Be x xex. Return to Exercise 5

26 Solutions to exercises 26 Exercise 6. y + y 2y = e 2x A.E. is m 2 + m 2 = 0 i.e. (m + 2)(m 1) = 0 i.e. m = 1 or m = 2 i.e. y CF = Ae x + Be 2x Try y P S = Ce 2x? No. This already appears in y CF Try y P S = Cxe 2x (i.e. multiply trial solution by x, until it does not appear in y CF ) i.e. y P S = 2Cxe 2x + Ce 2x = (1 2x)Ce 2x y P S = 2(1 2x)Ce 2x + ( 2)Ce 2x = ( 4 + 4x)Ce 2x

27 Solutions to exercises 27 Substitute: ( 4 + 4x)Ce 2x + (1 2x)Ce 2x 2Cxe 2x = e 2x Coeff. e 2x : 4C +4xC + C 2xC 2Cx = 1 i.e. C = 1 3, i.e. y P S = 1 3 xe 2x General solution is y = y CF + y P S = Ae x + Be 2x 1 3 xe 2x. Return to Exercise 6

28 Solutions to exercises 28 Exercise 7. y 2y + y = e x A.E. is m 2 2m + 1 = 0 i.e. (m 1) 2 = 0 i.e. m = 1 (twice) i.e. y CF = (A + Bx)e x Try y P S = Ce x? No, it s in y CF Try y P S = Cxe x? No. Also in y CF! Try y P S = Cx 2 e x. y P S = 2xCe x + Cx 2 e x, y P S = (2+2x)Ce x + (2x+x 2 )Ce x i.e. y P S = (2x + x 2 )Ce x = ( x 2 )Ce x Substitute: (2 + 4x + x 2 )Ce x 2(2x + x 2 )Ce x + Cx 2 e x = e x Coeff. e x : (2 + 4x + x 2 4x 2x 2 + x 2 )C = 1 i.e. C = 1 2 y P S = 1 2 x2 e x and general solution is y = y CF + y P S = (A + Bx)e x x2 e x. Return to Exercise 7

29 Solutions to exercises 29 Exercise 8. y + 8y + 17y = 2e 3x ; y(0) = 2 and y ( π 2 ) = 0 A.E. is m 2 + 8m + 17 = 0 i.e. m = 1 2 ( 8 ± ) = 4 ± i i.e. y CF = e 4x (A cos x + B sin x). f(x) = 2e 3x, so try y P S = Ce 3x i.e. y P S = 3Ce 3x, y P S = 9Ce 3x Substitute: 9Ce 3x + 8 ( 3)Ce 3x + 17Ce 3x = 2e 3x Coeff. e 3x : 9C 24C + 17C = 2 i.e. C = 1 y P S = e 3x, general solution y = y CF + y P S = e 4x (A cos x + B sin x) + e 3x.

30 Solutions to exercises 30 Now use the boundary conditions. y(0)=2 means y =2 when x=0 i.e. 2=e 0 (A cos 0+B sin 0)+e 0 i.e. 2 = A + 1 i.e. A=1 y( π 2 ) = 0 means y =0 when x= π 2 i.e. 0=e 2π (A cos π 2 +B sin π 2 )+e 3π 2 cos π 2 = 0 sin π 2 = 1 i.e. 0 = e 2π (0 + B) + e 3π 2 i.e. 0 = e 2π B + e 3π 2 (multiply both sides by e 2π ) i.e. 0 = B + e 3π 2 e 2π i.e. 0 = B + e π 2 i.e. B = e π 2 Required particular solution is y = e ( 4x cos x e π 2 sin x ) + e 3x. Return to Exercise 8

31 Solutions to exercises 31 Exercise 9. y + y 12y = 4e 2x ; y(0) = 7 and y (0) = 0 A.E. is m 2 + m 12 = 0 i.e. (m 3)(m + 4) = 0 i.e. m = 3 or m = 4 i.e. y CF = Ae 3x + Be 4x Try y P S = Ce 2x, y P S = 2Ce2x, y P S = 4Ce2x Substitute: 4Ce 2x + 2Ce 2x 12Ce 2x = 4e 2x Coeff. e 2x : 4C + 2C 12C = 4 i.e. C = 2 3 y P S = 2 3 e2x, general solution is y = Ae 3x + Be 4x 2 3 e2x.

32 Solutions to exercises 32 Apply boundary conditions to general solution (as always) y(0) = 7 means y = 7 when x = 0 i.e. 7 = Ae 0 + Be e0 i.e. 7 = A + B 2 3 i.e. 21 = 3A + 3B 2 (i) y (0) = 0 means y = 0 when x = 0, where y dy dx =3Ae3x 4Be 4x 4 3 e2x i.e. 0 = 3Ae 0 4Be e0 4 i.e. 3 = 3A 4B (ii)

33 Solutions to exercises 33 Solve equations (i) and (ii) simultaneously to find A and B (i)-(ii) gives = 7B 2 i.e. ( ) 1 7 = B 65 i.e = B i.e. B = 21, then (ii) gives 3A = B i.e = = 3A Particular solution is i.e. A = = y = 32 7 e3x e 4x 2 3 e2x. Return to Exercise 9

34 Solutions to exercises 34 Exercise 10. y + 3y + 2y = 10 cos 2x ; y(0) = 1 and y (0) = 0 A.E. is m 2 + 3m + 2 = 0 i.e. (m + 2)(m + 1) = 0 i.e. m = 2 or m 1 i.e. y CF = Ae 2x + Be x Try y P S = C cos 2x + D sin 2x i.e. y P S = 2C sin 2x + 2D cos 2x Substitute: y P S = 4C cos 2x 4D sin 2x 4C cos 2x 4D sin 2x + 3( 2C sin 2x + 2D cos 2x) + 2(C cos 2x + D sin 2x) = 10 cos 2x Coeff. cos 2x: 4C + 6D + 2C = 10 i.e. 2C + 6D = 10 (i) Coeff. sin 2x: 4D 6C + 2D = 0 i.e. 2D 6C = 0 (ii)

35 Solutions to exercises 35 Solve (i) and (ii) for C and D ( 3) times (i) gives +6C 18D = 30 (i) ADD 6C 2D = 0 (ii) 20D = 30 i.e. D = 3 2, (ii) then gives 2 ( 3 2) 6C = 0 i.e. 3 6C = 0 y P S = 1 2 cos 2x sin 2x ; i.e. C = 1 2. general solution is: y = Ae 2x + Be x 1 2 cos 2x sin 2x Boundary condition y(0) = 1 gives 1 = A + B 1 2 i.e. A+B = 3 2 (iii) y = 2Ae 2x Be x + sin 2x + 3 cos 2x y (0) = 0 gives 0 = 2A B + 3 i.e. 2A+B=3 (iv)

36 Solutions to exercises 36 Solve (iii) and (iv) to find A and B (iv)-(iii) gives A = = 3 2. Then, (iv) gives 2 ( 3 2) + B = 3 i.e. B = 0 Particular solution is y = 3 2 e 2x 1 2 cos 2x sin 2x. Return to Exercise 10

37 Solutions to exercises 37 Exercise 11. y + 4y + 5y = 2e 2x ; y(0) = 1 and y (0) = 2 A.E. is m 2 + 4m + 5 = 0 i.e. m = 1 2 i.e. y CF = e 2x (A cos x+b sin x) ( 4 ± ) = 2 ± i Try y P S = Ce 2x, y P S = 2Ce 2x, y P S = 4Ce 2x Substitute: 4Ce 2x 8Ce 2x + 5Ce 2x = 2e 2x Coeff. e 2x : 4C 8C + 5C = 2 i.e. C = 2. y P S = 2e 2x, general solution is y = e 2x (A cos x + B sin x) + 2e 2x

38 Solutions to exercises 38 Boundary condition y(0) = 1: 1 = A cos 0 + B sin i.e. 1 = A + 2 i.e. A = 1 y = 2e 2x (A cos x + B sin x) + e 2x ( A sin x + B cos x) 4e 2x y (0) = 2 gives 2 = 2 ( 1 + 0) + (0 + B) 4 i.e. 2 = 2 + B 4 i.e. B = 0. Particular solution is y = e 2x ( cos x + 0) + 2e 2x i.e. y = e 2x (2 cos x). Return to Exercise 11

39 Solutions to exercises 39 Exercise 12. d 2 x dτ + 4 dx 2 dτ + 3x = e 3τ ; x = 1 2 and dx dτ = 2 at τ = 0 A.E. is m 2 + 4m + 3 = 0 i.e. (m + 3)(m + 1) = 0 i.e. m = 3 or m = 1 i.e. x CF = Ae 3τ + Be τ Try x P S = Ce 3τ? No. Already in x CF Try x P S = Cτe 3τ, Substitute: dx P S dτ = Ce 3τ 3τCe 3τ = (1 3τ)Ce 3τ. d 2 x P S dτ 2 = 3Ce 3τ 3(1 3τ)Ce 3τ = (9τ 6)Ce 3τ (9τ 6)Ce 3τ + 4(1 3τ)Ce 3τ + 3Cτe 3τ = e 3τ Coeff. e 3τ : 9τC 6C + 4C 12τC + 3Cτ = 1 i.e. C = 1 2

40 Solutions to exercises 40 x P S = 1 2 τe 3τ, general solution is x = x CF + x P S = Ae 3τ + Be τ 1 2 τe 3τ Boundary conditions x = 1 2 when τ = 0 : 1 2 = A + B (i) dx dτ = 3Ae 3τ Be τ τe 3τ 1 2 e 3τ dx dτ = 2 when τ = 0 : 2 = 3A B 1 2 i.e. 3 2 = 3A B (ii) Solve (i) and (ii) for A and B: (i)+(ii) gives = A 3A i.e. A = 1 2 Then, (i) gives B = 0. Particular solution is x = 1 2 e 3τ 1 2 τe 3τ = 1 2 (1 τ)e 3τ. Return to Exercise 12

41 Solutions to exercises 41 Exercise 13. d 2 y dτ + 4 dy 2 dτ + 5y = 6 sin τ A.E. is m 2 + 4m + 5 = 0 i.e. m = 1 2 y CF = e 2τ (A cos τ +B sin τ) ( 4 ± ) = 2 ± i [Since e 2τ multiplies sin τ in y CF, sin τ is an independent function with respect to the components of y CF ]. Try y P S = C cos τ + D sin τ y P S = C sin τ + D cos τ = C cos τ D sin τ y P S d (each dash denoting dτ, here) Substitute: C cos τ D sin τ +4( C sin τ + D cos τ) +5(C cos τ + D sin τ) = 6 sin τ

42 Solutions to exercises 42 Coeff. cos τ: C + 4D + 5C = 0 i.e. C + D = 0 (i) Coeff. sin τ: D 4C + 5D = 6 i.e. C + D = 3 2 (ii) ADD 2D = 3 2 i.e. D = 3 4 and C = 3 4 General solution: y = e 2τ (A cos τ + B sin τ) 3 4 (cos τ sin τ). Return to Exercise 13

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