# Thread: Wronskian computation

1. ## Wronskian computation

Compute the Wronskian of the given set of functions:

cos ax, sin ax, x, a does not =0, in any interval.

Could someone help me with this?

Thanks,

Newt

2. Originally Posted by Jim Newt
Compute the Wronskian of the given set of functions:

cos ax, sin ax, x, a does not =0, in any interval.

Could someone help me with this?

Thanks,

Newt

$\begin{vmatrix}
\cos(ax) && \sin(ax) && x \\
-a \sin(ax) && a \cos(ax) && 1 \\
-a^2 \cos(ax) &&-a^2 \sin(ax) && 0 \\
\end{vmatrix}$

Now factor an $a^2$ out of row 3

$a^2 \begin{vmatrix}
\cos(ax) && \sin(ax) && x \\
-a \sin(ax) && a \cos(ax) && 1 \\
- \cos(ax) &&- \sin(ax) && 0 \\
\end{vmatrix}$

add row3 to row 1 and expand the det along the 1st row

$a^2 \begin{vmatrix}
0 && 0 && x \\
-a \sin(ax) && a \cos(ax) && 1 \\
- \cos(ax) &&- \sin(ax) && 0 \\
\end{vmatrix}= a^2 \cdot x \begin{vmatrix}
-a \sin(ax) && a \cos(ax) \\
-\cos(ax) && - \sin(ax) \\
\end{vmatrix}$

$=a^2 \cdot x (a \sin^{2}(ax)+a \cos^{2}(ax))=a^2 \cdot x \cdot (a)=a^3 \cdot x$

so the wronskian is not ALWAYS zero so the set is linearly independant.