Answer: To solve the third-order differential equation y''' + y' = cot(x) by variation of parameters, we first need to find the solution to the associated homogeneous equation, which is:
y''' + y' = 0
The characteristic equation is r^3 + r = 0, which can be factored as r(r^2 + 1) = 0. This gives us the roots r = 0, r = i, and r = -i. Therefore, the general solution to the homogeneous equation is:
y_h = c1 + c2 cos(x) + c3 sin(x)
To find a particular solution to the non-homogeneous equation using variation of parameters, we assume that the solution has the form:
y_p = u1(x) + u2(x) cos(x) + u3(x) sin(x)
where u1, u2, and u3 are functions to be determined.
We can find the derivatives of y_p:
y'_p = u1'(x) + u2'(x) cos(x) - u2(x) sin(x) + u3'(x) sin(x) + u3(x) cos(x)
y''_p = u1''(x) + u2''(x) cos(x) - 2u2'(x) sin(x) - u2(x) cos(x) + u3''(x) sin(x) + 2u3'(x) cos(x) - u3(x) sin(x)
y'''_p = u1'''(x) + u2'''(x) cos(x) - 3u2''(x) sin(x) - 3u2'(x) cos(x) - u2(x) sin(x) + u3'''(x) sin(x) + 3u3''(x) cos(x) - 3u3'(x) sin(x)
Substituting these derivatives into the non-homogeneous equation, we get:
u1'''(x) + u2'''(x) cos(x) - 3u2''(x) sin(x) - 3u2'(x) cos(x) - u2(x) sin(x) + u3'''(x) sin(x) + 3u3''(x) cos(x) - 3u3'(x) sin(x) + u1'(x) + u2'(x) cos(x) - u2(x) sin(x) + u3'(x) sin(x) + u3(x) cos(x) = cot(x)
Grouping the terms with the same functions together, we get:
u1'''(x) + u1'(x) = 0
u2'''(x) cos(x) - 3u2''(x) sin(x) - u2(x) sin(x) + u2'(x) cos(x) + u2'(x) cos(x) = cot(x) cos(x)
u3'''(x) sin(x) + 3u3''(x) cos(x) - 3u3'(x) sin(x) + u3'(x) sin(x) + u3(x) cos(x) = cot(x) sin(x)
The first equation is a first-order differential equation, which can be solved by integrating both sides:
u1'(x) + u1(x) = c1
where c1 is a constant of integration. The solution to this equation is:
u1(x) = c1 + c2 e^(-x)
where c2 is another constant of integration.
Explanation: