Revision: Diffrential Equations JEE Main Diffrential Equations

The highest exponent of the highest derivative is called the degree of a differential equation, provided exponents of each derivative and an unknown variable appearing in the differential equation are non-negative integers.

The order of the highest differential coefficient (or the highest order derivative appearing in a differential equation) is the order of the differential equation.

A solution of a differential equation in which the number of arbitrary constants equals the order of the differential equation is called the general solution of the differential equation.

Any relation between independent and dependent variables which does not involve derivatives, such that this relation and the derivatives obtained from it satisfy the given differential equation, is called a solution of the differential equation.

A solution obtained from the general solution by giving particular values to the arbitrary constants is called a particular solution.

A differential equation \[ \frac{dy}{dx} = F(x,y) \] is said to be in variable separable form if \[ F(x,y) = g(x)h(y). \]

Thus, \[ \frac{dy}{dx} = g(x)h(y) \] and, if \[ h(y) \neq 0, \]

\[ \boxed{\,\frac{1}{h(y)}\,dy = g(x)\,dx\,}. \]

A differential equation of the form \[\frac{dy}{dx}=\frac{f_{1}(x,y)}{f_{2}(x,y)},\] where f₁(x, y)  and f₂(x, y)  are homogeneous functions of x and y of the same degree, is called a homogeneous differential equation.

A function \[ F(x,y) \] is called homogeneous of degree \[ n \] if

\[ \boxed{\,F(\lambda x, \lambda y) = \lambda^{n} F(x,y)\,} \]

for any non-zero constant \[ \lambda. \]

A first-order and first-degree differential equation of the form

\[ \boxed{\,\frac{dy}{dx} + Py = Q\,} \]

where \[ P \] and \[ Q \] are constants or functions of \[ x \] only, is called a first-order linear differential equation.

Similarly, if \[ x \] is treated as a function of \[ y, \] it may be written as

\[ \boxed{\,\frac{dx}{dy} + P_{1}x = Q_{1}\,} \]

where \[ P_{1} \] and \[ Q_{1} \] are constants or functions of \[ y \] only.

For y = vx:

\[ \boxed{\,\frac{dy}{dx} = v + x\,\frac{dv}{dx}\,} \]

\[ \frac{dv}{g(v) - v} = \frac{dx}{x} \]

For x = vy:

\[ \boxed{\,\frac{dx}{dy} = v + y\,\frac{dv}{dy}\,} \]

• Order = highest derivative order.

• Degree = power of highest derivative.

• Degree exists only for polynomial equations in derivatives.

• Always check polynomial condition before stating the degree.

• Order and degree, if defined, are always positive integers.

1. Basic Idea:

• Form a differential equation from a given equation by eliminating arbitrary constants

2. Steps:

• Identify arbitrary constants in the given equation

• Differentiate the equation with respect to x as many times as the number of constants

• Eliminate constants from the obtained equations

3. Important Rule:

• Number of differentiations = number of arbitrary constants

4. Final Result:

• After eliminating constants → required differential equation is obtained

6. Important Note:

• A differential equation represents a family of curves

• A differential equation contains derivatives of an unknown function.

• Its solution is generally a function, not a single number.

• The graph of the solution function is called the solution curve or integral curve.

• A general solution contains arbitrary constants.

• A particular solution is obtained by assigning fixed values to those constants.

• To verify a solution, substitute the function and its derivatives into the equation and check whether LHS = RHS.

• A variable separable equation can be rearranged so that all y-terms are with dy and all x-terms are with dx.

• Separate variables first, then integrate.

• Use one constant of integration.

• Apply the initial condition only after getting the general solution.

• Final answers may be explicit or implicit.

• Check homogeneity first.

• Differentiate substitution carefully.

• Convert to separable form.

• Back-substitute to original variables.

1. Write the equation in the form dy/dx + Py = Q
2. Identify P and Q or P₁ and Q₁
3. Find I.F. = 

   \[ \frac{dy}{dx} + Py = Q \Rightarrow \text{I.F.} = e^{\int P\,dx} \]

   \[ \frac{dx}{dy} + P_{1}x = Q_{1} \Rightarrow \text{I.F.} = e^{\int P_{1}\,dy} \]

4. Multiply the whole equation by I.F.
5. Integrate and get a solution.