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- à 5.1èReal Roots-Third Order, Lïear, Constant Coefficient
- èè Differential Equation
- äèFïd ê general solution
-
- â y»»» - 6y»» + 11y» - 6y = 0
- The characteristic equation
- mÄ - 6mì + 11m - 6 = 0
- Facërs ïëè (m - 1)(m - 2)(m - 3) = 0
- The solutions areèm = 1, 2, 3
- The general solution is
- C¬e╣ + C½eì╣ + C¼eÄ╣
-
- éS è The LINEAR, HOMOGENEOUS, CONSTANT COEFFICIENT, THIRD ORDER
- DIFFERENTIAL EQUATION can be written ï ê form
- Ay»»» + By»» + Cy» + D = 0
- where A, B, C å D are constants.
- è As with ê correspondïg SECOND ORDER differential
- equation, an assumption is made that ê form ç ê solutions
- is
- y = e¡╣
- Differentiatïg å substitutïg yields
- (AmÄ + Bmì + Cm + D)e¡╣ = 0
- As e¡╣ is never zero, it can be cancelled yieldïg ê
- CHARACTERISTIC EQUATION
- AmÄ + Bmì + Cm + D = 0
-
- èèEvery CUBIC EQUATION with real coefficients has at least
- ONE REAL ROOT.èThe oêr two roots are eiêr
- a) BOTH REAL or
- b) a COMPLEX CONJUGATE PAIR
-
- èèIf all THREE ROOTS are REAL, êre are three subcases
- a) 3 distïct roots, say l, m, n
- The general solution is
- C¬e╚╣ + C½e¡╣ + C¼eⁿ╣
-
- b) 2 repeated roots, say m, m, å one oêr root,
- say n.èThe general solution is
- C¬e¡╣ + C½xe¡╣ + C¼eⁿ╣
-
- c) 3 repeated roots, say m, m, m
- The general solution is
- C¬e¡╣ + C½xe¡╣ + C¼xìe¡╣
-
- As with ê second order, non-homogeneous differential
- equation, solvïg a third order, NON-HOMOGENEOUS differential
- equation is done ï two parts.
-
- 1) Solve ê HOMOGENEOUS differential equation for a
- GENERAL SOLUTION with THREE ARBITRARY CONSTANTS
-
- 2) Fïd ANY PARTICULAR SOLUTION ç ê NON-HOMOGENEOUS
- differential equation.èAs disucssed ï CHAPTER 4, êre are
- two maï techniques for fïdïg a particular solution.
-
- A) METHOD OF UNDETERMINED COEFFICIENTS
- This technique is used when ê non-homogeneous
- term is
- 1)è A polynomial
- 2)è A real exponential
- 3)è A sïe or cosïe times a real exponential
- 4)è A lïear combïation ç ê above.
- This technique is explaïed ï à 4.3 å can be
- for ANY ORDER differential equation.
-
- B) METHOD OF VARIATION OF PARAMETERS
- This technique is valid for an ARBITRARY NON-HOMOGEN-
- EOUS TERM.èIt does require ê ability ë evaluate
- N ïtegrals for an Nth order differential equaën.
- As ê order ç ê differential equation ïcreses,
- ê ïtegrals become messier ï general.èThe second
- order version is discussed ï à 4.4.
-
- 1èè y»»» - 4y» = 0
-
- A)è C¬ + C½e╣ + C¼eúÅ╣
- B) C¬ + C½eúì╣ + C¼eì╣
- C)è C¬ + C½eú╣ + C¼eúÅ╣
- D)è C¬ + C½e╣ + C¼eÅ╣
-
- ü èèFor ê differential equation
- y»»» - 4y» = 0
- ê CHARACTERISTIC EQUATION is
- mÄ - 4m = 0
- This facërs ïë
- m(mì - 4) = 0
- å furêr ë
- m(m - 2)(m + 2) = 0
- This has ê solutions
- m = -2, 0, 2
- Thus ê general solution is
- C¬ + C½eúì╣ + C¼eì╣
-
- ÇèB
-
- 2 y»»» + 4y»» + y» - 6y = 0
-
- A)è C¬e╣ + C½ì╣ + C¼eÄ╣
- B)è C¬e╣ + C½ì╣ + C¼eúÄ╣
- C)è C¬e╣ + C½úì╣ + C¼eúÄ╣
- D)è C¬eú╣ + C½úì╣ + C¼eúÄ╣
-
- ü èèFor ê differential equation
- y»»» + 4y»» + y» - 6y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ + 4mì + m - 6 = 0
- This facërs ïë
- (m - 1)(m + 2)(m + 3) = 0
- This has ê solutions
- m = -3, -2, 1
- Thus ê general solution is
- C¬e╣ + C½eúì╣ + C¼eúÄ╣
-
- ÇèC
-
- è3 y»»» + 5y»» - 2y» + 10y = 0
-
- A)è C¬eÉ╣ + C½eú╣ + C¼eì╣
- B)è C¬eúÉ╣ + C½e╣ + C¼eúì╣
- C)è C¬eÉ╣ + C½eúá║ ╣ + C¼eáì ╣
- D)è C¬eúÉ╣ + C½eúá║ ╣ + C¼eáì ╣
- ü èèFor ê differential equation
- y»»» + 5y»» - 2y» + 10y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ + 5mì - 2m + 10 = 0
- This facërs ïë
- (mì - 2)(m + 5) = 0
- The first facër is IRREDUCIBLE over ê rationals but does
- facër over ê irrationals ë
- (m - √2)(m + √2)(m + 5) = 0
- This has ê solutions
- m =è-5,-√2, √2
- Thus ê general solution is
- C¬eúÉ╣ + C½eú√ì ╣ + C¼e√ì ╣
-
- ÇèD
-
- 4 6y»»» - 5y»» + y» = 0
-
- A)è C¬ + C½eì╣ + C¼eÄ╣ è
- B)è C¬ + C½eúì╣ + C¼eúÄ╣
- C)è C¬ + C½e╣»ì + C¼e╣»Ä
- D)è C¬ + C½eú╣»ì + C¼eú╣»Ä
- ü èèFor ê differential equation
- 6y»»» - 5y»» + y» = 0
- ê CHARACTERISTIC EQUATION is
- 6mÄ - 5mì + mè= 0
- This facërs ïë
- m(6mì - 5m + 1) = 0
- å furêr ë
- m(2m - 1)(3m - 1)
- This has ê solutions
- m = 0, 1/2, 1/3
- Thus ê general solution is
- C¬ + C½e╣»ì + C¼e╣»Ä
-
- ÇèC
-
- S 5 y»»» - 6y»» + 11y» - 6 = 0
-
- A)è C¬e╣ + C½eì╣ + C¼eÄ╣
- B)è C¬e╣ + C½eì╣ + C¼eúÄ╣
- C)è C¬e╣ + C½eúì╣ + C¼eúÄ╣
- D)è C¬eú╣ + C½eúì╣ + C¼eúÄ╣
- ü èèFor ê differential equation
- y»»» - 6y»» + 11y» - 6y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ - 6mì + 11m - 6 = 0
- This facërs ïë
- (m - 1)(m - 2)(m - 3) = 0
- This has ê solutions
- m =è1, 2, 3
- Thus ê general solution is
- C¬e╣ + C½eì╣ + C¼eÄ╣
-
- ÇèA
-
- 6 y»»» - 7y»» + 16y» - 12y = 0
-
- A) C¬eì╣ + C½xeì╣ + C¼eÄ╣
- B) C¬eì╣ + C½eúì╣ + C¼eÄ╣
- C) C¬eì╣ + C½eì╣ + C¼eúÄ╣
- D) C¬eì╣ + C½eúì╣ + C¼eúÄ╣
- ü èèFor ê differential equation
- y»»» - 7y»» + 16y» - 12y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ - 7mì + 16m - 12 = 0
- This facërs ïë
- (m - 2)(m - 2)(m - 3) = 0
- or
- (m - 2)ì(m - 3) = 0
- This has ê solutions
- m =è2, 2, 3
- Thus ê general solution is
- C¬eì╣ + C½xeì╣ + C¼eÄ╣
-
- ÇèA
-
- 7 12y»»» - 32y»» + 15y» + 9y = 0
-
- A) C¬eÄ╣»ì + C½xeÄ╣»ì + C¼e╣»Ä
- B) C¬eÄ╣»ì + C½xeÄ╣»ì + C¼eú╣»Ä
- C) C¬eúÄ╣»ì + C½xeúÄ╣»ì + C¼e╣»Ä
- D) C¬eúÄ╣»ì + C½xeúÄ╣»ì + C¼eú╣»Ä
-
- ü èèFor ê differential equation
- 12y»»» - 32y»» + 15y» + 9y = 0
- ê CHARACTERISTIC EQUATION is
- 12mÄ - 32mì + 15m + 9 = 0
- This facërs ïë
- (2m - 3)(2m - 3)(3m + 1) = 0
- or
- (2m - 3)ì(3m + 1) = 0
- This has ê solutions
- m =è3/2, 3/2, -1/3
- Thus ê general solution is
- C¬eÄ╣»ì + C½xeÄ»ì╣ + C¼eú╣»Ä
-
- ÇèB
-
- 8è y»»» + 9 y»» + 27y» + 27y = 0
-
- A) C¬e╣ + C½eÄ╣ + C¼eö╣
- B) C¬eú╣ + C½eúÄ╣ + C¼eúö╣
- C) C¬eÄ╣ + C½eÄ╣ + C¼eÄ╣
- D) C¬eúÄ╣ + C½xeúÄ╣ + C¼xìeúÄ╣
-
- ü èèFor ê differential equation
- y»»» + 9y»» + 27y» +27y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ + 9mì + 27m + 27 = 0
- This facërs ïë
- (m + 3)(m + 3)(m + 3) = 0
- or
- (m + 3)Ä = 0
- This has ê solutions
- m =è-3, -3, -3
- Thus ê general solution is
- C¬eúÄ╣ + C½xeúÄ╣ + C¼xìeúÄ╣
-
- ÇèD
-
- äèSolve ê ïitial value problem
-
- â è For ê Initial Value Problem,
- y»»» - 3y»» + 2y» = 0
- y(0) = 6, y»(0) = 4, y»»(0) = 10
- The general solution isè C¬ + C½e╣ + C¼eì╣
- Differentiatïg å substitutïg 0 for x produces a system ç
- three equations ï ê three constants.èSolvïg this system
- gives ê solutionèè y = 5 - 2e╣ + 3eì╣
-
- éSèèAs ê GENERAL SOLUTON ç a THIRD ORDER differential
- equation has THREE ARBITRARY CONSTANTS, for an Initial Value
- Problem ë completely specify which member ç this three
- parameter family ç curves requires INITAL VALUES.
- è The ståard ïitial values problem for a third order,
- lïear, constant coefficient differential equation is
- Ay»»» + By»» + Cy» + Dy = g(x)
- èèèy(x╠) =è y╠
- èè y»(x╠) =èy»╠
- èèy»»(x╠) = y»»╠
- èèAs with ê second order, ïital value problem, solvïg
- this problem is a 2 step process
-
- 1)èèSolve ê differential equation ë produce a general
- solution with three arbitrary constants.
-
- 2)èèCalculate ê first å second derivatives ç ê general
- solution.èThen substitue ê ïital value ç ïdependent
- variable, x╠ , ïë ê general solution å its first two
- derivatives.èThis will produce a system ç 3 equations ï
- ê three arbitrary constants.èSolvïg this system gives ê
- values ç ê three constants which gives ê specific
- solution ç ê ïitial value problem.
-
- 9è y»»» - 9y» = 0
- y(0) = 4èy»(0) = -12èy»»(0) = 18
-
- A) 2 + 3eúÄ╣ + e╣Ä╣
- B) 2 + 3eúÄ╣ - eÄ╣
- C) 2 - 3eúÄ╣ + e╣Ä╣
- D) -2 + 3eúÄ╣ + e╣Ä╣
-
- ü èèFor ê differential equation
- y»»» - 9y» = 0
- ê CHARACTERISTIC EQUATION is
- mÄ - 9m = 0
- This facërs ïë
- m(m - 3)(m + 3) = 0
- This has ê solutions
- m =è0, -3, 3
- Thus ê general solution is
- èy = C¬ + C½eúÄ╣ + C¼eÄ╣
- Differentiatïg
- y» = -3C½eúÄ╣ + 3C¼eÄ╣
- y»» =è9C½eúÄ╣ + 9C¼eÄ╣
- Substitutïg ê ïital value ç ê dependent variable 0
- èy(0) =è 4 = C¬ +èC½ +èC¼
- y»(0) = -12 =èè- 3C½ + 3C¼
- y»»(0) =è18 =èèè9C½ + 9C¼
- Sovlïg this system ç equations yields
- C¬ = 2è C½ = 3èC¼ = -1
- Thus ê solution ç ê ïitial value problem is
- y = 2 + 3eúÄ╣ - eÄ╣
- ÇèB
-
- 10 y»»» - 6y»» + 11y» - 6y = 0
- y(0) = -3èy»(0) = -8èy»»(0) = -26
-
- A) 2e╣ + 3eì╣ + 4eÄ╣
- B) 2e╣ - 3eì╣ + 4eÄ╣
- C) -2e╣ + 3eì╣ - 4eÄ╣
- D) -2e╣ - 3eì╣ - 4eÄ╣
-
- ü èèFor ê differential equation
- y»»» - 6y»» + 11y» - 6y = 0
- ê CHARACTERISTIC EQUATION is
- mÄ - 6mì + 11m -6 = 0
- This facërs ïë
- (m - 1)(m - 2)(m - 3) = 0
- This has ê solutions
- m =è1, 2, 3
- Thus ê general solution is
- èy = C¬e╣ + C½eì╣ + C¼eÄ╣
- Differentiatïg
- y» = C¬e╣ + 2C½eì╣ + 3C¼eÄ╣
- y»» = C¬e╣ + 4C½eì╣ + 9C¼eÄ╣
- Substitutïg ê ïital value ç ê dependent variable 0
- èy(0) =è-3 = C¬ +èC½ +èC¼
- y»(0) =è-8 = C¬ + 2C½ + 3C¼
- y»»(0) = -26 = C¬ + 4C½ + 9C¼
- Sovlïg this system ç equations yields
- C¬ = -2è C½ = 3èC¼ = -4
- Thus ê solution ç ê ïitial value problem is
- y = -2e╣ + 3eì╣ - 4eÄ╣
- ÇèC
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