 Solution

After multiplying either solution by $-1$ if necessary, suppose both are positive on $(x_1,x_2)$. SetThe equations giveSince ${\phi }_1$ is positive between consecutive zeros,ThereforeBut $W$ is nondecreasing, so both endpoint values coincide at zero and $W^{\prime }\equiv 0$. Since both solutions are strictly positive inside,This is the equality case in the wronskian proof of Sturm comparison.