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Methods of Solving Differential Equations> Variable Separable Differential Equations

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Estimated time: 9 minutes
CBSE: Class 12
Maharashtra State Board: Class 12

Definition: Equations in Variable Separable Form

The equation \[\frac{dy}{dx} = F(x, y)\] is said to be in variable separable form if it can be expressed as \[g(x) dx = h(y) dy\] or equivalently as \[\frac{dy}{dx} = g(x)h(y)\] so that the variables can be separated and integrated.

CBSE: Class 12

Method

  • Write the equation in the form \[\frac{dy}{dx} = g(x)h(y)\].

  • Rearrange it so that all y-terms are with dy, and all x-terms are with dx.

    \[\frac{1}{h(y)} dy = g(x) dx\]
  • Integrate both sides.

    \[\int \frac{1}{h(y)} dy = \int g(x) dx\]
  • Add the constant of integration.

    \[\int \frac{1}{h(y)} dy = \int g(x) dx + C\]
  • If an initial condition is given, substitute it to obtain the particular solution.

CBSE: Class 12

Example 1

Solve:

\[\frac{dy}{dx} = \frac{x + 1}{2 - y}\]

Solution:

Separate the variables:

(2 - y) dy = (x + 1) dx

Integrate both sides:

\[\int (2 - y) dy = \int (x + 1) dx\]
\[2y - \frac{y^2}{2} = \frac{x^2}{2} + x + C\]

On simplification, the general solution as

\[x^2 + y^2 + 2x - 4y + C = 0\]
CBSE: Class 12

Key Points: Variable Separable Differential Equations

  • Variable separable equations can be rewritten as x-part = y-part.

  • 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.

Example Question 1

Solve the following equation.

2x + 2 = 8

2x + 2 = 8

∴ 2x + 2 - 2 = 8 - 2

∴ 2x = 6

∴ x = 3

Example Question 2

Solve the following equation.

3x - 5 = x - 17

3x - 5 = x - 17

3x - 5 + 5 - x = x - 17 + 5 - x

∴ 2x = - 12

∴ x = - 6

Example Question 3

The length of a rectangle is 1 cm more than twice its breadth. If the perimeter of the rectangle is 50 cm, find its length.

Let the breadth of the rectangle be x cm.
Then the length of the rectangle will be (2x +1)cm.
2 × length + 2 × breadth = perimeter of rectangle

2 (2x + 1) + 2x = 50

∴ 4x + 2 + 2x = 50

∴ 6x + 2 = 50

∴ 6x = 50 - 2 = 48

∴ x = 8

Breadth of rectangle is 8 cm.

Length of the rectangle = 2x + 1 = 2 × 8 + 1

∴ Length of rectangle = 17 cm.

Example Question 4

Solve the following equation.

The sum of two consecutive natural numbers is 69. Find the numbers.

Let one natural number be x.
The next natural number is x + 1
(x) + (x + 1) = 69
∴ x + x + 1 = 69
∴ 2x + 1 = 69
2x = 69 - 1
∴ 2x = 68
∴ x = 34

1st natural number = 34
2nd natural number = 34 + 1 = 35.

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