bxcz yo zoo ba.c ca.b [bxaxcx aycy azcz cxaxbx ayby azbz x 0 y 0 z xaybxcy azbxcz aybycx azbzcx yo zoo they agree problem 1.6 axbxc x y z axbxc bxcxa cxa-xb so axbxc axbxc -bxcxa ba.c ca.b ca.b ac.b ab.c bc.a ab.c o ca.b if this is zero then either a is parallel to c including the case in which they point in oppositedirections or one is zero or else b.c b.a 0 in which case b is perpendicular to a