For a setAof metric space (X, d),

?A={x?X:??>0(B?(x)?A?=? ?B?(x)??A)}=

={x?X:??>0(B?(x)?A?=? ?B?(x)?(S\A)?=?)}

is called the boundary ofA.Consider the metricd(x, y) =|x1?y1|+|x2?y2|inR2and draw the set{x?R2: 0

2a. For a set A of metric space (X, d), = { (v p (xx ] q v 0 # v u ( x ) g) x} = is called the boundary of A. Consider the metric d(x, y) = (x1 - yl| + |x2 - y2) in R2 and draw the set {x E R2 : 0