The Wronskian of two functions $ f_1$ and $ f_2$ is defined as
\[
W (f_1, f_2 ) =
\left | {\begin{array}{cc}
f_1 & f_2 \\
f_{1}^{'} & f_{2}^{'} \\
\end{array} } \right |
\]
Using this definition, what then is the Wronskian determinant?
Then, if the determinant is never 0 on the interval, the functions are linearly independent.