Variation of parameters: Difference between revisions

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Description of method: Pulled equation numbering out of <math> tags and made them linkable
m General second-order equation: Delimited matrices with square brackets to visually distinguish from parentheses everywhere used for function calls
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We have the system of equations
 
:<math>\begin{pmatrixbmatrix}
u_1(x) & u_2(x) \\
u_1'(x) & u_2'(x) \end{pmatrixbmatrix}
\begin{pmatrixbmatrix}
A'(x) \\
B'(x)\end{pmatrixbmatrix} =
\begin{bmatrix} 0 \\ f \end{bmatrix}.</math>
\begin{pmatrix}
0\\
f\end{pmatrix}.</math>
 
Expanding,
 
:<math>\begin{pmatrixbmatrix}
A'(x)u_1(x)+B'(x)u_2(x)\\
A'(x)u_1'(x)+B'(x)u_2'(x) \end{pmatrixbmatrix}
= \begin{pmatrixbmatrix} 0\\f\end{pmatrixbmatrix}.</math>
 
So the above system determines precisely the conditions
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We seek ''A''(''x'') and ''B''(''x'') from these conditions, so, given
 
:<math>\begin{pmatrixbmatrix}
u_1(x) & u_2(x) \\
u_1'(x) & u_2'(x)
\end{pmatrixbmatrix}
\begin{pmatrixbmatrix}
A'(x) \\
B'(x)\end{pmatrixbmatrix} =
\begin{pmatrixbmatrix}
0\\
f\end{pmatrixbmatrix}</math>
 
we can solve for (''A''′(''x''), ''B''′(''x''))<sup>T</sup>, so
 
:<math>\begin{pmatrixbmatrix} A'(x) \\ B'(x) \end{bmatrix} =
\begin{pmatrixbmatrix}
A'(x) \\
B'(x)\end{pmatrix}=
\begin{pmatrix}
u_1(x) & u_2(x) \\
u_1'(x) & u_2'(x)
\end{pmatrixbmatrix}^{-1}
\begin{pmatrixbmatrix} 0\\ f \end{pmatrixbmatrix} =\frac{1}{W} \begin{pmatrixbmatrix}
u_2'(x) & -u_2(x) \\
-u_1'(x) & u_1(x) \end{pmatrixbmatrix}
\begin{pmatrixbmatrix} 0\\ f \end{pmatrixbmatrix},</math>
 
where ''W'' denotes the [[Wronskian]] of ''u''<sub>1</sub> and ''u''<sub>2</sub>. (We know that ''W'' is nonzero, from the assumption that ''u''<sub>1</sub> and ''u''<sub>2</sub> are linearly independent.) So,