Consider the differential equation, $x\frac{dy}{dx}=y(\log_e y-\log_e x + 1)$, then which of the following are true? (A) It is a linear differential equation Choose the correct answer from the options given below: |
(A), (D) and (E) only (A) and (D) only (B) and (C) only (B), (C) and (E) only |
(B), (C) and (E) only |
The correct answer is Option (4) → (B), (C) and (E) only Given differential equation: $x\frac{dy}{dx}=y(\log y-\log x+1)$ Rewrite: $\frac{1}{y}\frac{dy}{dx}=\frac{\log y-\log x+1}{x}$ Substitute $u=\frac{y}{x}$, so $y=ux$ and $\frac{dy}{dx}=u+x\frac{du}{dx}$ Substitute into the differential equation: $x(u+x\frac{du}{dx})=ux(\log u+1)$ $xu+x^{2}\frac{du}{dx}=ux\log u+ux$ $x^{2}\frac{du}{dx}=ux\log u$ $\frac{du}{dx}=\frac{u\log u}{x}$ $\frac{du}{u\log u}=\frac{dx}{x}$ Integrate: $\int \frac{du}{u\log u}=\int \frac{dx}{x}$ $\log (\log u)=\log x + C$ $\log \left(\log \frac{y}{x}\right)=\log x + C$ This matches option (C). Check linearity: equation is nonlinear → (A) false. Check homogeneity: it reduces to a function of $\frac{y}{x}$ → (B) true. Check particular solution for $y(1)=1$: $u=\frac{y}{x}=1$ gives $\log 1 =0$ satisfying the equation, so $y=x$ is valid → (E) true. Final answer: (B), (C), and (E) are correct |