Consider a triangle ABC
Here AB = Vector a
AC= Vector b
Let angle between
a and
b be

Drop a perpedicular from point B on AC, say it is BD
Length of the perpendicular = BD = a sin

Therefore, area of the triangle ABC= (1/2) base * altitude = (1/2)*AC * BD
area between vectors
a and
b = area of triangle ABC = (1/2)*b*a sin

area between vectors
a and
b =(1/2)*b*a sin

= (1/2) of magnitude of
(a x b)