20 17 winter vacation homework, seventh-grade mathematics, Volume I, Intersecting Lines and Parallel Lines II, Answers People's Education Edition, Grade One Mathematics winter vacation homework Reference Answers. Answer: 1. 1，C 2，30 3，AB //CD 4，B 5，360 6，B 7，80。

8, DA //CB 9, omitted 1 1, ∠AOC= 45, ∠ BOC = 135 12, omitted.

13, omitted 14, ∠ A = 60 15, omitted.