The four sides of the diamond are equal, and the diagonal lines are perpendicular to each other and equally divided. Draw a picture and see for yourself. Diamonds will be divided into four right angles with the same size by diagonal lines, which are a/2 cm and b/2 cm respectively. The length of the hypotenuse is obtained by Pythagorean theorem. The circumference is four times the length of the hypotenuse of the triangle. The calculation is simple. Do it yourself ~

2

A parallelogram and a quadrilateral with parallel and equal opposite sides are necessary and sufficient conditions, that is, if you know that a quadrilateral with parallel and equal opposite sides must be a parallelogram, and vice versa. In addition, parallelism and equality are transitive. From the conditions of two parallelograms, it is not difficult to conclude that the three segments of AD BC EF are all parallel and equal to each other. It can be proved from the fact that AD and EF are parallel and equal.