how do we get (Rth + Rl)^2 + (Xth + Xl)^2 in the denominator ?!

isn't (Zth+Zl)^2=(Rth+Rl)^2 + 2j (Rth+Rl)(Xth+Xl) - (Xth+Xl)^2 ?

 

 

The key is to note that

Z² is not the same as |Z|²

Try taking the magnitude of what you got and you will get their result, since

|Z²| = |Z|²