y,z为两个解,那么y''+q(x)y=0 z''+q(x)z=0
朗斯基行列式W
=|y z |
|y' z'|
=yz'-y'z
W'=y'z'+yz''-y''z-y'z'=yz''-y''z=y(-q(x)z)-(-q(x)y)z=0
所以w为常数