There are 6/8 of a cake leftover after a birthday party. How many 1/4 pieces can be made from the leftover cake?
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Not necessarily. 
and 
may be linearly dependent, so that their span forms a subspace of 
that does not contain every vector in .
For example, we could have
and 
. Any vector 
of the form 
, where 
, is impossible to obtain as a linear combination of these and , since
unless
and 
.
For example, we could have
unless