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 Page 1


 
 
 
 
 
DIFFERENTIAL EQUATIONS 
 
? The order of a deferential equation is the order of highest derivative appearing in it. 
? The degree of a differential equation is the degree of the highest derivative occurring in it, 
after the differential equation is expressed in a form free from radicals & fractions.  
 
 For equations of first order & first degree  
  
? Variable Separation method  
  Collect all function of x & dx on one side.  
  Collect all function of y & dy on other side.  
  like f(x) dx = g(y) dy  
  solution: ? ? ? ? f x dx g y dy c ??
??
   
 
? Exact differential equation  
  An equation of the form  
    M(x, y) dx + N (x, y) dy = 0  
  For equation to be exact. 
    
MN
yx
??
?
??
  ; then only this method can be applied.  
  The solution is  
    a = ?
??
M dx (termsofNnotcontainingx)dy 
 
? Integrating factors  
        An equation of the form  
    P(x, y) dx + Q (x, y) dy = 0 
       This can be reduced to exact form by multiplying both sides by IF.  
   If 
?? ??
?
??
??
??
1 P Q
Q y x
  is a function of x, then  
   R(x) = 
?? ??
??
??
??
??
1 P Q
Q y x
    
   Integrating Factor  
   IF = exp ? ?
? ?
R x dx
?
   
   Otherwise, if 
QP
1
P
xy
?? ??
?
??
??
??
 is a function of y  
Page 2


 
 
 
 
 
DIFFERENTIAL EQUATIONS 
 
? The order of a deferential equation is the order of highest derivative appearing in it. 
? The degree of a differential equation is the degree of the highest derivative occurring in it, 
after the differential equation is expressed in a form free from radicals & fractions.  
 
 For equations of first order & first degree  
  
? Variable Separation method  
  Collect all function of x & dx on one side.  
  Collect all function of y & dy on other side.  
  like f(x) dx = g(y) dy  
  solution: ? ? ? ? f x dx g y dy c ??
??
   
 
? Exact differential equation  
  An equation of the form  
    M(x, y) dx + N (x, y) dy = 0  
  For equation to be exact. 
    
MN
yx
??
?
??
  ; then only this method can be applied.  
  The solution is  
    a = ?
??
M dx (termsofNnotcontainingx)dy 
 
? Integrating factors  
        An equation of the form  
    P(x, y) dx + Q (x, y) dy = 0 
       This can be reduced to exact form by multiplying both sides by IF.  
   If 
?? ??
?
??
??
??
1 P Q
Q y x
  is a function of x, then  
   R(x) = 
?? ??
??
??
??
??
1 P Q
Q y x
    
   Integrating Factor  
   IF = exp ? ?
? ?
R x dx
?
   
   Otherwise, if 
QP
1
P
xy
?? ??
?
??
??
??
 is a function of y  
 
 
 
 
 
     S(y) = 
QP
1
P
xy
?? ??
?
??
??
??
  
   Integrating factor, IF = exp 
? ? ? ?
?
S y dy  
 
? Linear Differential Equations  
  An equation is linear if it can be written as:  
   
? ? ? ?
?? y' P x y r x   
  If r(x) = 0 ; equation is homogenous  
    else r(x) ? 0 ; equation is non-homogeneous  
   y(x) = 
? ? p x dx
ce
?
?
  is the solution for homogenous form  
    for non-homogenous form, h = ? ? P x dx
?
    
   ? ?
h h
y x e e rdx c
?
??
??
??
?
  
 
? Bernoulli’s equation  
  The equation ??
n
dy
Py Qy
dx
   
  Where P & Q are function of x  
  Divide both sides of the equation by 
n
y & put  
? ? 1  n
yz
?
?   
   
? ? ? ?
? ? ? ?
dz
P 1 n z Q 1 n
dx
   
  This is a linear equation & can be solved easily.  
   
? Clairaut’s equation  
  An equation of the form y = Px + f (P), is known as Clairaut’s equation where P = 
? ?
dy
dx
   
  The solution of this equation is  
   y = cx + f (c) where c = constant  
 
 
Linear Differential Equation of Higher Order  
 
 Constant coefficient differential equation  
  
n
n n1
n1
n
1
d y d y
k .............. k y X
dx dx
?
?
? ? ? ?   
Page 3


 
 
 
 
 
DIFFERENTIAL EQUATIONS 
 
? The order of a deferential equation is the order of highest derivative appearing in it. 
? The degree of a differential equation is the degree of the highest derivative occurring in it, 
after the differential equation is expressed in a form free from radicals & fractions.  
 
 For equations of first order & first degree  
  
? Variable Separation method  
  Collect all function of x & dx on one side.  
  Collect all function of y & dy on other side.  
  like f(x) dx = g(y) dy  
  solution: ? ? ? ? f x dx g y dy c ??
??
   
 
? Exact differential equation  
  An equation of the form  
    M(x, y) dx + N (x, y) dy = 0  
  For equation to be exact. 
    
MN
yx
??
?
??
  ; then only this method can be applied.  
  The solution is  
    a = ?
??
M dx (termsofNnotcontainingx)dy 
 
? Integrating factors  
        An equation of the form  
    P(x, y) dx + Q (x, y) dy = 0 
       This can be reduced to exact form by multiplying both sides by IF.  
   If 
?? ??
?
??
??
??
1 P Q
Q y x
  is a function of x, then  
   R(x) = 
?? ??
??
??
??
??
1 P Q
Q y x
    
   Integrating Factor  
   IF = exp ? ?
? ?
R x dx
?
   
   Otherwise, if 
QP
1
P
xy
?? ??
?
??
??
??
 is a function of y  
 
 
 
 
 
     S(y) = 
QP
1
P
xy
?? ??
?
??
??
??
  
   Integrating factor, IF = exp 
? ? ? ?
?
S y dy  
 
? Linear Differential Equations  
  An equation is linear if it can be written as:  
   
? ? ? ?
?? y' P x y r x   
  If r(x) = 0 ; equation is homogenous  
    else r(x) ? 0 ; equation is non-homogeneous  
   y(x) = 
? ? p x dx
ce
?
?
  is the solution for homogenous form  
    for non-homogenous form, h = ? ? P x dx
?
    
   ? ?
h h
y x e e rdx c
?
??
??
??
?
  
 
? Bernoulli’s equation  
  The equation ??
n
dy
Py Qy
dx
   
  Where P & Q are function of x  
  Divide both sides of the equation by 
n
y & put  
? ? 1  n
yz
?
?   
   
? ? ? ?
? ? ? ?
dz
P 1 n z Q 1 n
dx
   
  This is a linear equation & can be solved easily.  
   
? Clairaut’s equation  
  An equation of the form y = Px + f (P), is known as Clairaut’s equation where P = 
? ?
dy
dx
   
  The solution of this equation is  
   y = cx + f (c) where c = constant  
 
 
Linear Differential Equation of Higher Order  
 
 Constant coefficient differential equation  
  
n
n n1
n1
n
1
d y d y
k .............. k y X
dx dx
?
?
? ? ? ?   
 
 
 
 
 
  Where X is a function of x only  
a. If 
n 12
y , y ,.........., y are n independent solution, then  
   
nn 1 1 2 2
c y c y .......... c y x ? ? ? ? is complete solution  
   where 
1 2 n
c ,c ,..........,c  are arbitrary constants.  
 
b. The procedure of finding solution of n
th
 order differential equation involves 
computing complementary function (C. F) and particular Integral (P. I).  
 
c. Complementary function is solution of  
  
n
n n1
n1
n
1
d y d y
k ............. k y 0
dx dx
?
?
? ? ? ?   
 
d. Particular integral is particular solution of  
  
n
n n1
n1
n
1
d y d y
k ............ k y x
dx dx
?
?
? ? ? ?  
 
e. y = CF + PI is complete solution  
 
 
Finding complementary function  
? Method of differential operator  
Replace 
d
dx
 by D ?
dy
Dy
dx
?   
Similarly  
      
n
n
d
dx
  by 
n
D ?  
n
n
n
dy
Dy
dx
?   
   
n
n n1
n1
n
1
d y d y
k ............ k y 0
dx dx
?
?
? ? ? ?  becomes  
   
? ?
n n 1
n
1
D k D ........... k y 0
?
? ? ? ?   
   Let 
12 n
m ,m ,............,m  be roots of  
   
n
1
n1
n
D k D ................ K 0
?
? ? ?   ………….(i) 
   
   
 
 
Page 4


 
 
 
 
 
DIFFERENTIAL EQUATIONS 
 
? The order of a deferential equation is the order of highest derivative appearing in it. 
? The degree of a differential equation is the degree of the highest derivative occurring in it, 
after the differential equation is expressed in a form free from radicals & fractions.  
 
 For equations of first order & first degree  
  
? Variable Separation method  
  Collect all function of x & dx on one side.  
  Collect all function of y & dy on other side.  
  like f(x) dx = g(y) dy  
  solution: ? ? ? ? f x dx g y dy c ??
??
   
 
? Exact differential equation  
  An equation of the form  
    M(x, y) dx + N (x, y) dy = 0  
  For equation to be exact. 
    
MN
yx
??
?
??
  ; then only this method can be applied.  
  The solution is  
    a = ?
??
M dx (termsofNnotcontainingx)dy 
 
? Integrating factors  
        An equation of the form  
    P(x, y) dx + Q (x, y) dy = 0 
       This can be reduced to exact form by multiplying both sides by IF.  
   If 
?? ??
?
??
??
??
1 P Q
Q y x
  is a function of x, then  
   R(x) = 
?? ??
??
??
??
??
1 P Q
Q y x
    
   Integrating Factor  
   IF = exp ? ?
? ?
R x dx
?
   
   Otherwise, if 
QP
1
P
xy
?? ??
?
??
??
??
 is a function of y  
 
 
 
 
 
     S(y) = 
QP
1
P
xy
?? ??
?
??
??
??
  
   Integrating factor, IF = exp 
? ? ? ?
?
S y dy  
 
? Linear Differential Equations  
  An equation is linear if it can be written as:  
   
? ? ? ?
?? y' P x y r x   
  If r(x) = 0 ; equation is homogenous  
    else r(x) ? 0 ; equation is non-homogeneous  
   y(x) = 
? ? p x dx
ce
?
?
  is the solution for homogenous form  
    for non-homogenous form, h = ? ? P x dx
?
    
   ? ?
h h
y x e e rdx c
?
??
??
??
?
  
 
? Bernoulli’s equation  
  The equation ??
n
dy
Py Qy
dx
   
  Where P & Q are function of x  
  Divide both sides of the equation by 
n
y & put  
? ? 1  n
yz
?
?   
   
? ? ? ?
? ? ? ?
dz
P 1 n z Q 1 n
dx
   
  This is a linear equation & can be solved easily.  
   
? Clairaut’s equation  
  An equation of the form y = Px + f (P), is known as Clairaut’s equation where P = 
? ?
dy
dx
   
  The solution of this equation is  
   y = cx + f (c) where c = constant  
 
 
Linear Differential Equation of Higher Order  
 
 Constant coefficient differential equation  
  
n
n n1
n1
n
1
d y d y
k .............. k y X
dx dx
?
?
? ? ? ?   
 
 
 
 
 
  Where X is a function of x only  
a. If 
n 12
y , y ,.........., y are n independent solution, then  
   
nn 1 1 2 2
c y c y .......... c y x ? ? ? ? is complete solution  
   where 
1 2 n
c ,c ,..........,c  are arbitrary constants.  
 
b. The procedure of finding solution of n
th
 order differential equation involves 
computing complementary function (C. F) and particular Integral (P. I).  
 
c. Complementary function is solution of  
  
n
n n1
n1
n
1
d y d y
k ............. k y 0
dx dx
?
?
? ? ? ?   
 
d. Particular integral is particular solution of  
  
n
n n1
n1
n
1
d y d y
k ............ k y x
dx dx
?
?
? ? ? ?  
 
e. y = CF + PI is complete solution  
 
 
Finding complementary function  
? Method of differential operator  
Replace 
d
dx
 by D ?
dy
Dy
dx
?   
Similarly  
      
n
n
d
dx
  by 
n
D ?  
n
n
n
dy
Dy
dx
?   
   
n
n n1
n1
n
1
d y d y
k ............ k y 0
dx dx
?
?
? ? ? ?  becomes  
   
? ?
n n 1
n
1
D k D ........... k y 0
?
? ? ? ?   
   Let 
12 n
m ,m ,............,m  be roots of  
   
n
1
n1
n
D k D ................ K 0
?
? ? ?   ………….(i) 
   
   
 
 
 
 
 
 
 
 
           Case I: All roots are real & distinct  
    
? ? ? ? ? ?
n 12
D m D m ............ D m 0 ? ? ? ?    is equivalent to (i)  
     y = 
1 2n
mx mx mx
n
12
c e c e ........... c e ? ? ?   
    is solution of differential equation 
  Case II: If two roots are real & equal  
    i.e., 
12
m m m ??   
    y = 
? ?
2
3 n
mx
mx mx
n
13
c c x e c e .......... c e ? ? ? ?    
 
 
  Case III: If two roots are complex conjugate  
    
1
m j ? ? ? ?  ;  
2
m j ? ? ? ?   
    y = 
?
?? ? ? ? ? ?
?? 12
n
mx
n
x
e c 'cos x c 'sin x .......... c e   
 
 Finding particular integral  
   Suppose differential equation is  
   
n n1
n
n 1
n1
d y d y
k .......... k y X
dx
dx
?
?
? ? ? ?  
  Particular Integral  
   PI = 
? ?
? ?
? ?
? ?
? ?
? ?
? ? ? ? ? ?
? ? ?
n 12
n 12
W x W x W x
y dx y dx .......... y dx
W x W x W x
   
  Where 
n
12
y ,y ,............y are solutions of Homogenous from of differential equations.  
 
 
  ? ?
? ? ? ? ? ?
n
12
n
12
nn n
n
12
y y y
y ' y ' y '
Wx
y y y
?
?
?
?
?
       ? ?
? ? ? ? ? ? ? ?
n 12 i1
n 12 i1
i
n n n n
n 12 i1
y y y0y
y ' y ' y '0 y '
Wx
0
y y y 1y
?
?
?
? ?
?
?
?
?
     
 
 
  
? ?
i
Wx is obtained from W(x) by replacing i
th
 column by all zeroes & last 1.  
 
 
Page 5


 
 
 
 
 
DIFFERENTIAL EQUATIONS 
 
? The order of a deferential equation is the order of highest derivative appearing in it. 
? The degree of a differential equation is the degree of the highest derivative occurring in it, 
after the differential equation is expressed in a form free from radicals & fractions.  
 
 For equations of first order & first degree  
  
? Variable Separation method  
  Collect all function of x & dx on one side.  
  Collect all function of y & dy on other side.  
  like f(x) dx = g(y) dy  
  solution: ? ? ? ? f x dx g y dy c ??
??
   
 
? Exact differential equation  
  An equation of the form  
    M(x, y) dx + N (x, y) dy = 0  
  For equation to be exact. 
    
MN
yx
??
?
??
  ; then only this method can be applied.  
  The solution is  
    a = ?
??
M dx (termsofNnotcontainingx)dy 
 
? Integrating factors  
        An equation of the form  
    P(x, y) dx + Q (x, y) dy = 0 
       This can be reduced to exact form by multiplying both sides by IF.  
   If 
?? ??
?
??
??
??
1 P Q
Q y x
  is a function of x, then  
   R(x) = 
?? ??
??
??
??
??
1 P Q
Q y x
    
   Integrating Factor  
   IF = exp ? ?
? ?
R x dx
?
   
   Otherwise, if 
QP
1
P
xy
?? ??
?
??
??
??
 is a function of y  
 
 
 
 
 
     S(y) = 
QP
1
P
xy
?? ??
?
??
??
??
  
   Integrating factor, IF = exp 
? ? ? ?
?
S y dy  
 
? Linear Differential Equations  
  An equation is linear if it can be written as:  
   
? ? ? ?
?? y' P x y r x   
  If r(x) = 0 ; equation is homogenous  
    else r(x) ? 0 ; equation is non-homogeneous  
   y(x) = 
? ? p x dx
ce
?
?
  is the solution for homogenous form  
    for non-homogenous form, h = ? ? P x dx
?
    
   ? ?
h h
y x e e rdx c
?
??
??
??
?
  
 
? Bernoulli’s equation  
  The equation ??
n
dy
Py Qy
dx
   
  Where P & Q are function of x  
  Divide both sides of the equation by 
n
y & put  
? ? 1  n
yz
?
?   
   
? ? ? ?
? ? ? ?
dz
P 1 n z Q 1 n
dx
   
  This is a linear equation & can be solved easily.  
   
? Clairaut’s equation  
  An equation of the form y = Px + f (P), is known as Clairaut’s equation where P = 
? ?
dy
dx
   
  The solution of this equation is  
   y = cx + f (c) where c = constant  
 
 
Linear Differential Equation of Higher Order  
 
 Constant coefficient differential equation  
  
n
n n1
n1
n
1
d y d y
k .............. k y X
dx dx
?
?
? ? ? ?   
 
 
 
 
 
  Where X is a function of x only  
a. If 
n 12
y , y ,.........., y are n independent solution, then  
   
nn 1 1 2 2
c y c y .......... c y x ? ? ? ? is complete solution  
   where 
1 2 n
c ,c ,..........,c  are arbitrary constants.  
 
b. The procedure of finding solution of n
th
 order differential equation involves 
computing complementary function (C. F) and particular Integral (P. I).  
 
c. Complementary function is solution of  
  
n
n n1
n1
n
1
d y d y
k ............. k y 0
dx dx
?
?
? ? ? ?   
 
d. Particular integral is particular solution of  
  
n
n n1
n1
n
1
d y d y
k ............ k y x
dx dx
?
?
? ? ? ?  
 
e. y = CF + PI is complete solution  
 
 
Finding complementary function  
? Method of differential operator  
Replace 
d
dx
 by D ?
dy
Dy
dx
?   
Similarly  
      
n
n
d
dx
  by 
n
D ?  
n
n
n
dy
Dy
dx
?   
   
n
n n1
n1
n
1
d y d y
k ............ k y 0
dx dx
?
?
? ? ? ?  becomes  
   
? ?
n n 1
n
1
D k D ........... k y 0
?
? ? ? ?   
   Let 
12 n
m ,m ,............,m  be roots of  
   
n
1
n1
n
D k D ................ K 0
?
? ? ?   ………….(i) 
   
   
 
 
 
 
 
 
 
 
           Case I: All roots are real & distinct  
    
? ? ? ? ? ?
n 12
D m D m ............ D m 0 ? ? ? ?    is equivalent to (i)  
     y = 
1 2n
mx mx mx
n
12
c e c e ........... c e ? ? ?   
    is solution of differential equation 
  Case II: If two roots are real & equal  
    i.e., 
12
m m m ??   
    y = 
? ?
2
3 n
mx
mx mx
n
13
c c x e c e .......... c e ? ? ? ?    
 
 
  Case III: If two roots are complex conjugate  
    
1
m j ? ? ? ?  ;  
2
m j ? ? ? ?   
    y = 
?
?? ? ? ? ? ?
?? 12
n
mx
n
x
e c 'cos x c 'sin x .......... c e   
 
 Finding particular integral  
   Suppose differential equation is  
   
n n1
n
n 1
n1
d y d y
k .......... k y X
dx
dx
?
?
? ? ? ?  
  Particular Integral  
   PI = 
? ?
? ?
? ?
? ?
? ?
? ?
? ? ? ? ? ?
? ? ?
n 12
n 12
W x W x W x
y dx y dx .......... y dx
W x W x W x
   
  Where 
n
12
y ,y ,............y are solutions of Homogenous from of differential equations.  
 
 
  ? ?
? ? ? ? ? ?
n
12
n
12
nn n
n
12
y y y
y ' y ' y '
Wx
y y y
?
?
?
?
?
       ? ?
? ? ? ? ? ? ? ?
n 12 i1
n 12 i1
i
n n n n
n 12 i1
y y y0y
y ' y ' y '0 y '
Wx
0
y y y 1y
?
?
?
? ?
?
?
?
?
     
 
 
  
? ?
i
Wx is obtained from W(x) by replacing i
th
 column by all zeroes & last 1.  
 
 
 
 
 
 
 
 
 Euler-Cauchy Equation  
  An equation of the form  
   
n n 1
n n 1
n
n 1
n1
d y d y
x k x .......... k y 0
dx
dx
?
?
?
? ? ? ?    
  is called as Euler-Cauchy theorem  
  Substitute y = x
m
  
  The equation becomes  
   
? ? ? ? ? ?
??
? ? ? ? ? ? ? ? ?
??
m
n 1
m m 1 ........ m n k m(m 1)......... m n 1 ............. k x 0   
  The roots of equation are  
 
  Case I: All roots are real & distinct  
    
1 2 n
mm
m
n
12
y c x c x ........... c x ? ? ? ?   
  
    Case II: Two roots are real & equal  
    
12
m m m ??   
                         
? ?
3
m
m m
n
1 2 3
n
y c c nx x c x ........ c x ? ? ? ? ?  
 
    Case III:  Two roots are complex conjugate of each other  
    
1
mj ? ? ? ? ;   
2
mj ? ? ? ?    
    y = 
? ? ? ?
?
??
? ? ? ? ? ? ?
??
3
m
n
m
n 3
x Acos nx Bsin nx c x ........... c x     
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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