在$\mathit{\Delta ABC}$和△$A_1{B}_1{C}_1$中，已知$\angle C=\angle C_1=90°$，$\mathit{AC}=A_1{C}_1=3$，$\mathit{BC}=4$，$B_1{C}_1=2$，点$D$、$D_1$分别在边$\mathit{AB}$、$A_1{B}_1$上，且$\mathit{\Delta ACD}\cong$△$C_1{A}_1{D}_1$，那么$\mathit{AD}$的长是　　．