2. Yes. At this time, its four corners are right angles.

3. yes. At this time, it gets an equilateral quadrilateral with right angles.

4. Angle A=60 degrees, angle B=30 degrees.

5.( 1) angle BAD=60, angle ABC= 120.

(2)AB = 6 cm, AC = 10.39 cm.

6. We can know AB=CD by angle ABD= angle DBC= angle ADB, and AB=BC in the same way, so that AD is parallel to BC, and the quadrilateral ABCD is a diamond.

7.45 degrees

8. Rectangular. All four corners are right angles.

9.45 degrees.

10. Quadrilateral Amen Amen, EFCG are quadrilaterals with equal sides.

11.ab× DH = 2ao× od = 2aAbd area, and DH=2.4CM can be obtained.

12.B(- root number12,0), C (0 0,2), d (root number12,0)

13. The quadrilateral EFMN is a square.

14.3 species.

15.ADE are all equal to BAF.

16.BO = 2OD, which must be passed.

17. There are countless kinds. Just keep the two paths perpendicular to each other and cross the center of the square.