This is a little bit like asking why a square has four sides. A regular pentagon is defined in such a way as to require that it has 5-fold rotational symmetry.
It has rotational symmetry of order 2.
Yes, it does.
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Parallelograms: 2-fold Square: 4-fold n-fold symmetries refer to rotational symmetries. Consequently, any symmetries about axes that these and other quadrilaterals may have are not relevant to this question.
A design with four-fold symmetry can be rotated 90, 180, or 270 degrees and still maintain all of its characteristics. This means there are three different places it can be rotated while keeping its symmetry.
This is a little bit like asking why a square has four sides. A regular pentagon is defined in such a way as to require that it has 5-fold rotational symmetry.
Because it only has four sides. So it can only have two lines of symmetry because if you fold it the sides will be the same shape.
A two-fold symmetry has a 360 degrees rotation. A three-fold rotational symmetry, on the other hand, has 120 degrees, and on a horizontal axis, a symmetry has 180 degrees.
A cube has 3-fold rotational symmetry, meaning it can be rotated by 120 degrees and still look the same. It does not have 5-fold rotational symmetry because the cube's faces are not oriented in a way that allows for that type of symmetry. The angles between the faces do not align with the requirements for 5-fold rotational symmetry.
It has rotational symmetry of order 2.
4
two-fold
Yes, it does.
line
Yes, it does.
Sting rays have a two fold axis of symmetry which would make it closed symmetry.