Beam collapse mechanism - Steel

I had a question about the material in the Structural Engineering Reference Manual 8th edition. I am referring to example 4.29 on page 4-42. The work shown for sway mechanism and combined mechanisms doesn’t match the formulas used to get them: sway Mp = P1 x h / 4 and for combined sway it’s Mp = ((P1 x h) + (P2 x l)/2) /6.  Is this an error on example 4.29? I checked the errata but I didn’t find anything posted on this matter.  

Thanks.