The set M(2,2) of a 2x2 matrix, with real entries, is a vector space.
The set of diagonal matrices
D=(a 0 )
( 0 b )
where a, b are real numbers. is a subset of M(2,2).
Write down two particular matrices which belong to D, and two particular matrices which belong to M(2,2) but not D.
Two matrices which belong to D are
v =(0) u=(b)
I don't get the other part.. which does not belong to D.