Short Answer Type
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.
If either vector 
But the converse need not be true. Justify your answer with an example.
Switch
if for a unit vector 
is the unit vector and 
then find 
and 
then what can be concluded about the vector 
?
and 
if 
and 
show that 
are 
then show that the vectors 
perpendicular to each other?
