Constructing a second solution

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SUMMARY

The forum discussion focuses on finding a second solution to the differential equation xy" + y' = 0, given the first solution y1 = ln(x). The user attempts to derive the second solution y2 using the formula y2 = y1(x) ( ∫(e-∫P(x)dx) / (y1(x)²) )dx, where P(x) = 1/x. However, there are errors in the integration process and the application of the formula, leading to confusion regarding the validity of the derived solution y2 = 1. The discussion highlights the importance of correctly applying integration techniques in solving differential equations.

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1. Find a second solution of the differential eq. by using the formula.

xy" + y' = 0 ; y1 = ln(x)


2.

y2 = y1(x) ( ∫(e-∫P(x)dx) / (y1(x)2) )dx





3.

I found the p(x):

p(x) = 1/x


and then I plug in everything into the formula:

y2 = ln(x) ∫(e-∫((1)/(x))dx) / (ln(x)2 )dx



solve:


= ln(x) ( ∫(e^(-ln(x))) / (x)(ln(x)2))



= ∫ (1)/ (x)(ln(x))


I do not know if this is correct. I'm going to need some help.

the answer is y2 = 1


 
Last edited:
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Hang on, if P(x) = 1/x, why did you plug in ln(x) in the exponential integral term?

Edit: Also you can't multiply ln(x) into the integral.
 
Last edited:
Mugged said:
Hang on, if P(x) = 1/x, why did you plug in ln(x) in the exponential integral term?

Edit: Also you can't multiply ln(x) into the integral.

sorry. typo. the ln(x) was after integration of (1/x)
 
Rework the integral, i don't understand where you get that x in the denominator. An answer of y2 = 1 seems trivial..y2 could be any constant and it would satisfy the differential equation.
 

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