I believe this is a typo, unless I’m missing something.
I believe between lines 2 and 3, you are factoring out a u(Xi) from the last two terms, and then further pulling out a -1 from both terms in parens to set up the next line.
But that -1 then disappears when it shouldn’t, and so I believe the minus between the fractions in the last line should actually be a plus.
I believe this is a typo, unless I’m missing something.
I believe between lines 2 and 3, you are factoring out a u(Xi) from the last two terms, and then further pulling out a -1 from both terms in parens to set up the next line.
But that -1 then disappears when it shouldn’t, and so I believe the minus between the fractions in the last line should actually be a plus.