10 1620 degrees 1 1 4cm or 16cm 12, 16 13, root

14,10cm, 5cm, 5 numbers, 3cm.

15,5cm162.517,60 degrees

18 ( 1)(2), (2)(4)

19, the root number 3 BD is easy to find, and it is easier for you to do it by equal area method.

DF multiplied by CB=BD multiplied by the congruence of distance triangle DEF and CFB, and the length of DF is obtained by Pythagorean theorem.

20, 3 root number 3 translates a waist, which is easy.

2 1 The answer is not unique.

22. The quadrilateral CDC prime number E is a diamond.

Even CC apostrophe, folding, DE bisects CC apostrophe vertically, so CD=DC apostrophe, you just need to do that parallelogram.

23 is the midpoint.

(2) It is a rectangle, and the three lines of AD are in one line, so it is perpendicular to BC.

24.，( 1) BE=DF+EF

(2) DF=BE+EF

(3) EF=BE+DF