2

$-\frac{\lambda }{2}+\frac{\sqrt{-{\Delta }_1}}{2}\frac{-c_1\sin \left(\frac{\sqrt{-{\Delta }_1}}{2}\theta \right)+c_2\cos \left(\frac{\sqrt{-{\Delta }_1}}{2}\theta \right)}{c_2\sin \left(\frac{\sqrt{-{\Delta }_1}}{2}\theta \right)+c_1\cos \left(\frac{\sqrt{-{\Delta }_1}}{2}\theta \right)}$

${\Delta }_1<0$

3

$-\frac{\lambda }{2}+\frac{c_2}{c_1+c_2\theta }$

${\Delta }_1=0$