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21.如图,已知$EA⊥ AB$于点A,$CD⊥ DF$于点D,$AB// CD$,那么EA与DF平行吗?为什么?
答案
21.$EA//DF$.理由略.
22.小熙和小组同学根据平行线的知识开展课题学习活动. 如图，$∠ CDF + ∠ DFE = 180°$，$∠ C = ∠ DAE$，求证:$AD// BC$.
答案
22.$\because ∠ CDF+∠ DFE=180°$,
$\therefore EF//CD$.
$\therefore ∠ C=∠ AEB$.
$\because ∠ C=∠ DAE$,
$\therefore ∠ AEB=∠ DAE$.
$\therefore AD//BC$.
$\therefore EF//CD$.
$\therefore ∠ C=∠ AEB$.
$\because ∠ C=∠ DAE$,
$\therefore ∠ AEB=∠ DAE$.
$\therefore AD//BC$.
