A regular four-sided pyramid has an edge of the base plane of length 2. The adjacent side edges of the pyramid are enclosed at an angle of 
.
Express the surface area of the pyramid in terms of 
.
Calculate the value of , where the volume of the pyramid is 
.
Show that the volume of the pyramid is 
.
Let 
. Show that at this angle the opposite side edges of the pyramid are perpendicular.
