Question 98: As shown in the figure, in the tetrahedron $\mathrm{D}-\mathrm{ABC}$, it is known that $\mathrm{AB}=2, \overrightarrow{\mathrm{AC}} \cdot \overrightarrow{\mathrm{BD}}=-3$, let $\mathrm{AD}=\mathrm{a}, \mathrm{BC}=b$, $\mathrm{CD}=\mathrm{c}$, try to find the minimum value of $\frac{\mathrm{c}^{2}}{\mathrm{ab}+1}$.