A square loop of wire in the x-y plane, centred on the origin, is made up of four straight segments, each of length 2d=10cm. A current I =10A flows in the wire in the direction shown. Find the magnetic field at z=d on the z-axis. (See "Diagram" for the diagram of the problem) Here is the solution provided by the professor: (See "solution1" for the solution of the problem) I don't understand how he came up with the expression for unite vectors a1 and a3. Could some one explain this? Thanks.