5. 如图，点A在射线DF上，点C在射线BE上，$∠ B+∠ BAD=180°$，$∠ 1=∠ 2$.试说明：$AB// CD$.
答案：5. $\because ∠ B+∠ BAD=180°$,
$∠ 1+∠ BAD=180°$,
$\therefore ∠ 1=∠ B$.
$\because ∠ 1=∠ 2$,
$\therefore ∠ 2=∠ B$.
$\therefore AB// CD$.
$∠ 1+∠ BAD=180°$,
$\therefore ∠ 1=∠ B$.
$\because ∠ 1=∠ 2$,
$\therefore ∠ 2=∠ B$.
$\therefore AB// CD$.
1. 如图，已知直线$a// b$，$∠ 2=120°$，则$∠ 1$的度数为(
A.$55°$
B.$60°$
C.$120°$
D.$125°$
B
).A.$55°$
B.$60°$
C.$120°$
D.$125°$
答案：1. B.