In tetrahedron ABCD altitudes AA', BB',CC',DD' intersects in one point. O is a centre of sphere circumscribed on ABCD.Prove that if DO is perpendicular to plane A'B'C' then triangle ABC is equilateral. Please hint.
In tetrahedron ABCD altitudes AA', BB',CC',DD' intersects in one point. O is a centre of sphere circumscribed on ABCD.Prove that if DO is perpendicular to plane A'B'C' then triangle ABC is equilateral. Please hint.
A hint? Okay... if the altitudes of the tetrahedron all intersect in one point, what is that telling you about the tetrahedron?