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,mszeptuch,fedora,10.03.2024 20:52,file:///home/mszeptuch/.config/libreoffice/4; |
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## Równania liniowe | ||
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### Równania liniowe 1 rzędu | ||
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$$ | ||
y' + p(x) * y = f(x) \\ | ||
niech~ | ||
L(y) = y' + p(x) * y \\ | ||
L(y_1 + y_2) = L(y_1) + L(y_2) \\ | ||
L(\alpha y) = \alpha L(y) \\ | ||
$$ | ||
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Mówi się, że równanie jest liniowe, jeżeli _lewa strona_ jest liniowa | ||
ze względu na `y`. | ||
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```{note} | ||
jeżeli $f(x) = 0$, mówimy o równaniu jednorodnym. | ||
``` | ||
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```{tip} | ||
niech $y^{*}$ i $y$ będą rozwiązaniami równania liniowego niejednorodnego, wtedy | ||
$$ | ||
L(y^{*}) - L(y) = f(x) - f(x) = 0 \\ | ||
L(y^{*} - y) = 0 | ||
$$ | ||
Z tego wynika, że różnica $y^{*} - y$ jest rozwiązaniem równania jednorodnego | ||
``` | ||
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```{admonition} tw. kukurydzy | ||
$$ | ||
CORN = CSRN + CORJ | ||
$$ | ||
- CORN = całka ogólna równania niejednorodnego | ||
- CSRN = Całka szczególna równania jednorodnego | ||
- CORJ = Całka ogólna równania jednorodnego | ||
``` | ||
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$$ | ||
y' + p(x) y = 0 \\ | ||
\frac{dy}{y} = -p(x) dx \\ | ||
\int \frac{dy}{y} = \int -p(x) dx \\ | ||
ln(y) = \int p(x) dx + C \\ | ||
|y| = e^{\int p(x) dx} + C \\ | ||
CORJ: \quad y = Ce^{\int p(x) dx} \\ | ||
$$ | ||
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#### Metoda Uzmienniania stałej: | ||
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Szuikamy CSRN W postaci $y(x) = C(x) * e^{-\int p(x) dx}$ | ||
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$$ | ||
C'(x) * e^{-\int p(x) dx} + \cancel{C(x) * (-p(x)) e^{-\int p(x) dx}} + \cancel{p(x)* C(x)e^{-\int p(x) dx}} = f(x) \\ | ||
C'(x) * e^{-\int p(x) dx} = f(x) \\ | ||
C'(x) = f(x) e^{\int p(x) dx} | ||
C(x) = \int f(x) e^{\int p(x) dx} | ||
$$ | ||
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#### Metoda Przewidywania | ||
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Jeżeli $p(x) = const$ | ||
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$$ | ||
\frac{dy}{dx} + 3y = x^2 \\ | ||
$$ | ||
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Pomińmy COFJ, CSRN: | ||
Rozwiązaniem Najprawodopodobniej będzie wielomian stopnia 2 | ||
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$$ | ||
y=Ax^2 + Bx + C \\ | ||
y' = 2Ax + b \\ | ||
2Ax + B + 3Ax^2 + 3Bx + 3C = x^2 \\ | ||
\left\{\begin{matrix} | ||
3A = 1 \\ | ||
2A + 3B = 0 \\ | ||
B + 3C = 0 \\ | ||
\end{matrix}\right. \Rightarrow | ||
\left\{\begin{matrix} | ||
A = \frac{1}{3} \\ | ||
B = \frac{-2}{9} \\ | ||
3C = \frac{2}{27} \\ | ||
\end{matrix}\right. | ||
$$ | ||
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Jeżeli f(x) jest w postaci funkcji trygonometrycznych, zakładamy | ||
rozwiązanie w postaci $Asin(x) + Bcos(x)$ |