Q.41 The solution to dy/dx + y cot x = cosec x is y = (c + x) cot x y = (c + x) cosec x y = (c + x) cosec x cot x y = (c + x) cosec x / cot x

Q.41 The solution to

dy/dx + y cot x = cosec x

is

  1. y = (c + x) cot x
  2. y = (c + x) cosec x
  3. y = (c + x) cosec x cot x
  4. y = (c + x) cosec x / cot x

Step-by-Step Solution

Step 1: Identify the Standard Form

The given equation is a first-order linear differential equation of the form:

dy/dx + P(x)y = Q(x)

Here,

P(x) = cot x

Q(x) = cosec x

Step 2: Find the Integrating Factor (IF)

Integrating Factor (IF) = e∫P(x) dx

IF = e∫cot x dx

∫cot x dx = ln(sin x)

IF = eln(sin x) = sin x

Step 3: Multiply the Equation by IF

sin x · dy/dx + y sin x cot x = sin x cosec x

Since:

sin x cot x = cos x

sin x cosec x = 1

The equation becomes:

d/dx (y sin x) = 1

Step 4: Integrate Both Sides

∫ d/dx (y sin x) dx = ∫ 1 dx

y sin x = x + c

Step 5: Solve for y

y = (x + c) / sin x

y = (c + x) cosec x

Correct Answer

Option (B):
y = (c + x) cosec x

Explanation of All Options

Option (A): y = (c + x) cot x
Incorrect. This form does not satisfy the final integrated equation.

Option (B): y = (c + x) cosec x
Correct. This exactly matches the derived solution.

Option (C): y = (c + x) cosec x cot x
Incorrect. An extra cot x term is unnecessarily introduced.

Option (D): y = (c + x) cosec x / cot x
Incorrect. This simplifies to y = (c + x) sec x, which does not satisfy the equation.

Conclusion

The solution of the differential equation dy/dx + y cot x = cosec x using the integrating factor method is:

y = (c + x) cosec x

 

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