devarshi-dt-logo

Question:

If y(x) is the solution to the differential equation dydx + (2x + 1/x)y = e-x, x > 0, where y(1) = 1/2e-1, then:

y(loge2)=loge24

y(x)is decreasing in(0,1)

y(x)is decreasing in(1/2,1)

y(loge2)=loge4

Solution:

The given differential equation is:
dy/dx + (2x + 1/x)y = e-x
This is a linear differential equation of the form dy/dx + P(x)y = Q(x), where P(x) = 2x + 1/x and Q(x) = e-x.
The integrating factor is given by:
IF = e∫P(x)dx = e∫(2x + 1/x)dx = ex² + ln|x| = ex² * eln|x| = xex² (since x > 0)
Multiplying the differential equation by the integrating factor, we get:
x ex² dy/dx + (2x² + x/x) ex² y = xex²-x
x ex² dy/dx + (2x² + 1) ex² y = xex²-x
d/dx (xex² y) = xex²-x
Integrating both sides with respect to x, we get:
xex² y = ∫xex²-x dx + C
Let's solve the integral using integration by parts. Let u = x and dv = ex²-x dx. Then du = dx and v is difficult to find explicitly. Let's try another approach.
Let's rewrite the equation as:
dy/dx + (2x + 1/x)y = e-x
This is a first order linear ODE. The integrating factor is:
IF = exp(∫(2x + 1/x)dx) = exp(x² + ln x) = x exp(x²)
Multiplying by IF, we have:
x exp(x²) dy/dx + (2x² + 1) exp(x²) y = x exp(x² - x)
d/dx [x exp(x²) y] = x exp(x² - x)
Integrating both sides:
x exp(x²) y = ∫x exp(x² - x) dx + C
This integral is difficult to solve analytically. However, we are given an initial condition y(1) = 1/(2e).
Let's use this initial condition to find C.
When x = 1, y = 1/(2e):
e y = ∫01 t exp(t² - t) dt + C
We are given that y(1) = 1/(2e). Substituting this into the equation above, we can find C. However, solving the integral analytically is complicated and likely not necessary to determine which options are correct. We need more information or a different approach to solve this problem completely. The provided solution is incomplete and requires further analysis to determine the correct options.