Let A(a→) and B(b→) be points on two skew lines r→=a→+λp→ and  r→=b→+uq→ and the shortest distance between the skew lines is l, where p→ and q→ are unit vectors forming adjacent sides of a parallelogram enclosing an area of ! units. If an angle between l8 and the line of shortest distance is 60o, then AB =