Using the projective property cos= projective area/oblique area

So cos30= 1/S

S= root number 3/2

Similarly, there is S= root 2/2.

s= 1/2

These three aspects add up to:

S= (root number 3+ root number 2+ 1 )/2

The projective theorem is that the side area is greater than the whole bottom area = cos.

It is proved that when the intersection lines of two faces are perpendicular to each other, there can be a dihedral angle, which is set as a.

So there are:

Cos(a)= side height/bottom height.

= side area/total bottom area (because the intersection lines are equal and they are the same bottom).

The projected area is the projected area of the bottom edge.