解：?$(1)$?∵?$A$?和?$A'$?關(guān)于?$l_{1}$?對(duì)稱
∴?$A'$?應(yīng)在?$l_{1}$?的左邊，且到?$l_{1}$?的距離等于?$A$?到?$l_{1}$?的距離
?$ (2) $?如圖，?$A''$?應(yīng)在和?$A'$?關(guān)于?$l_{2}$?對(duì)稱的位置
?$ (3) $?設(shè)?$l_{1}$?與?$l_{2}$?之間的距離為?$a，$??$A'$?到?$l_{1}$?的距離為?$A'C，$?
?$A'$?到?$l_{2}$?的距離為?$A'D$?
∵?$A$?和?$A'$?關(guān)于?$l_{1}$?對(duì)稱，?$A'$?和?$A''$?關(guān)于?$l_{2}$?對(duì)稱
∴?$AA' = 2\ \mathrm {A}'C，$??$A'A'' = 2\ \mathrm {A}'D$?
則?$AA'' = A'A'' - AA' = 2(A'D - A'C)$?
而?$A'D - A'C$?就是?$l_{1}$?與?$l_{2}$?之間的距離?$a$?
∴?$AA'' = 2a$?
