Question
Given two points A and B, identify the set of points P in space such that
Solution
We have
Draw a sphere with O as centre and AB as diameter. Take any point P in the interior of this sphere and join PA and PB. It is clear that P is an interior point of the sphere if and only if ∠APB is obtuse.
∴ from (1), it follows that the required set of points is the interior of the sphere with AB as a diameter.