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A three-sided pyramid 
is given.
In triangle 
with data
and the radius of the triangle of the delineated circle 
, the point 
is the center of this triangle of the delineated circle. Calculate the scalar product 
.
Express the vectors 
and 
as a linear combination of vectors
Prove that the equality 
holds.
Let the pyramid be equal. Prove that the opposite edges 
and 
are perpendicular to each other.
