?$\text { 解: (1) } $?∵?$ l_1\ \mathrm {/} / l_2 $?
∴?$∠{ABD}+∠{BAC}=180° $?
∵?${AB} \perp l_2 $?
∴?$∠{ABD}=90° $?
∴?$∠{BAC}=90° $?
∴?${AB} \perp l_1 $?
?$(2)$?∵?${AB} \perp I_2， {CD} \perp l_2$?
∴?$∠{ABD}=∠{CDB}=90° $?
∵?$l_1\ \mathrm {/} / l_2 $?
∴?$∠{BAC}+∠{ABD}=∠{ACD}+∠{CDB}=180° $?
∴?$∠{BAC}=∠{ACD}=90° $?
∴?$\text { 四邊形ABCD是矩形 } $?
∴?${AB}={CD} .$?
