1.

Find the general solution by the method of variation of parameters.x2y " − 4xy' + 6y = x (Be sure to express the equation in standard form before setting up variation of parameters.)

Answer»

First we have to solve the homogeneous equation 

(3.1)  x2y" − 4xy' + 6y = 0. 

This is an Euler-Cauchy equation, so we look for solutions of the form y = xm. The characteristic equation for m is 

m(m − 1) − 4m + 6 = 0. 

Note m(m − 1) − 4m + 6 = m2 − 5m + 6 = (m − 2)(m − 3), so the roots are m = 2 and m = 3. Thus, we have basic solutions y1 = x2 and y2 = x3 for the homogeneous equation (3.1). 

To solve the equation in the problem, we put it in standard form by dividing both sides by x2. This gives us

y" -/xy' +  6/x2y = 1/x.

We look for a particular solution of this equation of the form y = u(x)y1(x)+ v(x)y2(x). The equations for u and v are 

u'x2 + v'x3 = 0 

2u'x + 3v'x2 = 1/x. 

If we multiply the top equation by −2 and the bottom equation by x, the system becomes 

−2u'x2 − 2v'x3 = 0 

2u'x2 + 3v'x3 = 1.

Adding these equations gives v'x3 = 1, so v' = 1/x3. Substituting this is the first equation gives u' = −1/x2. Easy integrations then give

u = 1/x

v = − 1/2x2 

Plugging into the formula y = u(x)y1(x) + v(x)y2(x) for our particular solution gives 

y = (1/x)x2 − (1/2x2)x3 = 1/2x. 

Adding on the general solution of the homogeneous equation, we find 

y = 1/2x + C1x2 + C2x3 

for the general solution of the inhomogeneous equation.



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