The quadrilateral EFGH is a square.

Because △ AEH △ BFE △ CGF △ DHG (corner)

It is known that the four sides of the quadrilateral EFGH are equal, that is, the quadrilateral is a diamond.

And ∠AEH =∠DHG∠AEH+∠AHE = 90,

So: ∠ dhg+∠ ahe = 90,

So: ∠ ehg = 90.

So: quadrilateral EFGH is a square.

Knowledge and skills 2

It is a square.

Because the diagonal of the quadrilateral ABCD is divided into two,

So: the quadrilateral ABCD is a parallelogram.

Because: AC=BD=(√2)AB.

So: the quadrilateral ABCD is a rectangle.

Because: OA? =OB？ =( 1/2)AB，

So: OA? +OB？ =AB？

So: △ABO is a right triangle, which is AC⊥BD.

So: AB=BC=CD=DA

So: quadrilateral ABCD is a square.