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(D²-2D+1)y=cos3x equation

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Numerical solution:

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The solution

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/ 2          \             
\d  - 2*d + 1/*y = cos(3*x)
$$y \left(\left(d^{2} - 2 d\right) + 1\right) = \cos{\left(3 x \right)}$$
Detail solution
Given the equation
$$y \left(\left(d^{2} - 2 d\right) + 1\right) = \cos{\left(3 x \right)}$$
transform
$$y \left(d^{2} - 2 d + 1\right) - \cos{\left(3 x \right)} - 1 = 0$$
$$y \left(\left(d^{2} - 2 d\right) + 1\right) - \cos{\left(3 x \right)} - 1 = 0$$
Do replacement
$$w = \cos{\left(3 x \right)}$$
Expand brackets in the left part
-1 - w + d+2+2*d+1y = 0

Looking for similar summands in the left part:
-1 - w + y*(1 + d^2 - 2*d) = 0

Move free summands (without w)
from left part to right part, we given:
$$- w + y \left(d^{2} - 2 d + 1\right) = 1$$
Divide both parts of the equation by (-w + y*(1 + d^2 - 2*d))/w
w = 1 / ((-w + y*(1 + d^2 - 2*d))/w)

We get the answer: w = -1 + y + y*d^2 - 2*d*y
do backward replacement
$$\cos{\left(3 x \right)} = w$$
substitute w:
The solution of the parametric equation
Given the equation with a parameter:
$$y \left(d^{2} - 2 d + 1\right) = \cos{\left(3 x \right)}$$
Коэффициент при y равен
$$d^{2} - 2 d + 1$$
then possible cases for d :
$$d < 1$$
$$d = 1$$
Consider all cases in more detail:
With
$$d < 1$$
the equation
$$y - \cos{\left(3 x \right)} = 0$$
its solution
$$y = \cos{\left(3 x \right)}$$
With
$$d = 1$$
the equation
$$- \cos{\left(3 x \right)} = 0$$
its solution
The graph
Rapid solution [src]
         /  cos(3*x)  \     /  cos(3*x)  \
y1 = I*im|------------| + re|------------|
         |     2      |     |     2      |
         \1 + d  - 2*d/     \1 + d  - 2*d/
$$y_{1} = \operatorname{re}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)} + i \operatorname{im}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)}$$
y1 = re(cos(3*x)/(d^2 - 2*d + 1)) + i*im(cos(3*x)/(d^2 - 2*d + 1))
Sum and product of roots [src]
sum
    /  cos(3*x)  \     /  cos(3*x)  \
I*im|------------| + re|------------|
    |     2      |     |     2      |
    \1 + d  - 2*d/     \1 + d  - 2*d/
$$\operatorname{re}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)} + i \operatorname{im}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)}$$
=
    /  cos(3*x)  \     /  cos(3*x)  \
I*im|------------| + re|------------|
    |     2      |     |     2      |
    \1 + d  - 2*d/     \1 + d  - 2*d/
$$\operatorname{re}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)} + i \operatorname{im}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)}$$
product
    /  cos(3*x)  \     /  cos(3*x)  \
I*im|------------| + re|------------|
    |     2      |     |     2      |
    \1 + d  - 2*d/     \1 + d  - 2*d/
$$\operatorname{re}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)} + i \operatorname{im}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)}$$
=
    /  cos(3*x)  \     /  cos(3*x)  \
I*im|------------| + re|------------|
    |     2      |     |     2      |
    \1 + d  - 2*d/     \1 + d  - 2*d/
$$\operatorname{re}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)} + i \operatorname{im}{\left(\frac{\cos{\left(3 x \right)}}{d^{2} - 2 d + 1}\right)}$$
i*im(cos(3*x)/(1 + d^2 - 2*d)) + re(cos(3*x)/(1 + d^2 - 2*d))