For the differential equation \(\left(x\log_e x\right)dy=\left(\log_e x-y\right)dx\)
(A) Degree of the given differential equation is 1.
(B) It is a homogeneous differential equation.
(C) Solution is \(2y\log_e x+A=\left(\log_e x\right)^2\), where A is an arbitrary constant
(D) Solution is \(2y\log_e x+A=\log_e\left(\log_e x\right)\), where A is an arbitrary constant
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
Dividing by \(x\log_e x\):
\( \displaystyle \frac{dy}{dx} = \frac{\log_e x - y}{x\log_e x} \)
\( \displaystyle \frac{dy}{dx} + \frac{1}{x\log_e x}\,y = \frac{1}{x} \)
This is a linear differential equation with \(P = \frac{1}{x\log_e x}\) and \(Q = \frac{1}{x}\). Since \(\frac{d}{dx}\left(\log_e(\log_e x)\right) = \frac{1}{x\log_e x}\), the integrating factor is
\( \displaystyle \text{I.F.} = e^{\int\frac{dx}{x\log_e x}} = e^{\log_e(\log_e x)} = \log_e x \)
The solution is \(y \times \text{I.F.} = \int Q \times \text{I.F.}\,dx\):
\( \displaystyle y\log_e x = \int\frac{\log_e x}{x}\,dx \)
\( \displaystyle y\log_e x = \frac{(\log_e x)^2}{2} + C \)
\(2y\log_e x - 2C = (\log_e x)^2\)
So the equation has order 1 and degree 1, its right side \(\frac{\log_e x - y}{x\log_e x}\) is not a function of \(\frac{y}{x}\) alone, and its solution is \(2y\log_e x + A = (\log_e x)^2\) with A = −2C.
Checking the statements
(A) Degree of the given differential equation is 1. — Correct. The only derivative, dy/dx, occurs to the power 1.
(B) It is a homogeneous differential equation. — Incorrect. Replacing x, y by λx, λy turns \(\log_e x\) into \(\log_e x + \log_e\lambda\), so the right side is not of degree zero.
(C) Solution is \(2y\log_e x+A=\left(\log_e x\right)^2\), where A is an arbitrary constant — Correct. It is the solution above with A = −2C.
(D) Solution is \(2y\log_e x+A=\log_e\left(\log_e x\right)\), where A is an arbitrary constant — Incorrect. The right side of the solution is \((\log_e x)^2\), not \(\log_e(\log_e x)\).