From the definition of the dot product, we can make certain useful observations about its properties.
(i) The angle
between two vectors
and
is given by
(ii)
, the equality holding only if
or 
(iii) The projection of
on
is
(iv) The projection of
on
is
(v) Scalar product is commutative i.e.,
(vi) Scalar product is distributive i.e.,
and |
(vi) The scalar product of two vectors is zero if and only if the two vectors are perpendicular.
This also gives
(vii) For any vector 
(viii) 
(ix) This property is very important. If two vectors
and
have been specified in rectangular form
We have
The angle
between the two vectors will be given by 
(x) The direction cosines
,
,
of a vector
will be given by
(xi) Let
be a vector coplanar with the vectors
and
. If
and
this would imply that
is perpendicular to both
and
. This can only happen if
and
are collinear.
Analogously, let
be an arbitrary vector and
be three vectors such that
This means that
is perpendicular to each of
,
and
which can only happen if
and
are coplanar.
(xii) Let
be three non-coplanar vectors. We’ve already discussed that
can form a basis for
space. Any vector
can be written in this basis as
This representation is of significant importance and you must understand how it comes about.
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