解：分情況討論直線?$a$?與?$a'$?的交角?$∠1，$??$∠2$?與旋轉(zhuǎn)角?$\theta$?的關(guān)系如下：
?$ (1) $?當(dāng)?$0°\leq \theta<90°$?時(shí)，銳角夾角?$∠1 = \theta ，$?鈍角夾角?$∠2 = 180°-\theta ；$?
?$ (2) $?當(dāng)?$\theta = 90°$?時(shí)，?$∠1=∠2 = 90°；$?
?$ (3) $?當(dāng)?$90°<\theta <180°$?時(shí)，銳角夾角?$∠1 = 180°-\theta ，$?鈍角夾角?$∠2 = \theta ；$?
?$ (4) $?當(dāng)?$\theta = 180°$?時(shí)，夾角為?$0°；$?
?$ (5) $?當(dāng)?$180°<\theta <270°$?時(shí)，銳角夾角?$∠1 = \theta - 180°，$?鈍角夾角?$∠2 = 360°-\theta ；$?
?$ (6) $?當(dāng)?$\theta = 270°$?時(shí)，?$∠1=∠2 = 90°；$?
?$ (7) $?當(dāng)?$270°<\theta <360°$?時(shí)，銳角夾角?$∠1 = 360°-\theta ，$?鈍角夾角?$∠2 = \theta - 180°。$?
