a (b) Bdi=uo(I1-I B di=o Bdi=uo(I-I)=0I I (c) l 3 I (b) l 2 I 2 I 1 (a) l 1 l 1  B.dl = I 1 I 2 (a) μ o ( ) l 2 (b)  B.dl = 0 l 3 (c)  B.dl =μ o (I I ) = 0