Question

If y(x) be the solution of the differential equation \(x \log(x) \frac{dy}{dx} + y = 2x \log(x), \quad y(e)\) is equal to

• A. e
• B. 2
• C. 0
• D. 2e

Answer 1

The Correct Option is B

The equation can be rewritten as:
\(\frac{dy}{dx} + \frac{1}{x} y = 2 \log x\)
Comparing this with the standard form \(\frac{dy}{dx} + P(x)y = Q(x)\)
we have 
\(P(x) = \frac{1}{x} \quad \text{and} \quad Q(x) = 2\log x\)

To find the integrating factor (IF), we calculate the exponential of the integral of 
\(P(x): \text{IF} = e^{\int \frac{1}{x} \, dx} = e^{\ln|x|} = |x|\)
Multiplying both sides of the differential equation by the integrating factor, we get: 
\(|x| \frac{dy}{dx} + \frac{y}{x} = 2 \log x |x|\)
Now, the left side of the equation is the derivative of the product\( (y|x|) \) with respect to x: 
\(\frac{d}{dx}(y|x|) = 2\log x |x|\)
Integrating both sides, we have:
\(y|x| = \int 2\log x |x| \, dx\)
Using integration by parts, we can evaluate the integral on the right side: 
\(y|x| = 2 \int \log x \, d\left(\frac{x^2}{2}\right) - 2 \int d(\log x) \cdot \frac{x^2}{2} \, dx \, y|x|\)

= \(2 \left[\frac{x^2}{2} \log x - \int \frac{x}{x^2} \cdot \frac{x^2}{2} \, dx \right] y|x|\)

=\(x^2 \log x - \int x \, dx \, y|x|\)

= \(x^2 \log x - \frac{x^2}{2} + C\)
To find the value of C, we can use the initial condition 
\(y(e) = y(2.71828) = 2: 2 \)
= \((2.71828^2) \log 2.71828 - \frac{(2.71828^2)}{2} + C \times 2\)

= \((2.71828^2) - \frac{(2.71828^2)}{2} + C \times 2\)

= \(\frac{(2.71828^2)}{2} + C \times C\)
= \(2 - \frac{(2.71828^2)}{2} \times C\)
\(≈ -1.34738\)

 Therefore, the solution to the differential equation is: 
\(y|x| = x^2 \log x - \frac{x^2}{2} - 1.34738\)
 To find y(e), we substitute 
\(x = e: \quad y(e) = (e^2) \log e - \frac{e^2}{2} - 1.34738 \, y(e)\)

= \(e^2 - \frac{e^2}{2} - 1.34738 \, y(e)\)

= \(\frac{e^2}{2} - 1.34738\)

So, \(y(e) \approx \frac{(2.71828^2)}{2} - 1.34738\) \(\text{which is approximately } 2.35067\)
Therefore, the value of y(e) is approximately 2.35067, which corresponds to option (B) 2.