? AD ∨ BC,? A=？ c，？ B=？ d，

A+？ B= 180？ ,

∵? A= 120？ ，B=60？ ,

C=？ A= 120？ ,? D=？ B=60？ .

2. Solution: Cause: In △ABC and △DBC, let BC be the base and the height be the distance between two parallel lines, and the distance between parallel lines is equal everywhere. So according to the area formula of triangle, we can know that the areas of these two triangles are equal.

Take any point on the straight line l 1 that is different from A and D, and the area of the triangle connected with B and C is equal to the area of △ABC.

Methods: The distance between two parallel lines is equal, which is an important property. It is often used when making auxiliary lines, so pay attention to understanding and mastering.

3. Solution: Because the perimeter of this parallelogram is 36cm and the sum of its two sides is 18cm, and because the length of each side of the parallelogram is a multiple of 3, its two sides can be 3 cm or 15cm, 6cm or 12cm, 9cm or 9cm, so the length of each side of this parallelogram is respectively. 15cm or 6cm, 12cm, 6cm, 12cm or 9cm, 9cm, 9cm, 9cm.

Bian Xiao suggested: