1.Given a regular tetrahedron ABCD
with vertices A(0,0,0)B(a,0,0),
C(0,a,0),and D(0,0,a).Calculate the
volume V and the surface area S of
this tetrahedron.
at what times, if exist, are the
angles betwen the hour hand, the
minute hand and the second hand
of a clock exactly 120°?
assume that the hands of the clock
move uniformly.
In a convex quadrilateral ABCD, diagonals AC and BD intersect at E, while perpendicular bisectors of AB and CD intersect at F, and those of BC and DA intersect at G. Prove: (1) E, F, and G are collinear, (2) AE:EC = BF:FD, and (3) CG:GD = AF:FB.