Im Have this equation and I have the result but I canīt get from a to b....
There are several errors in your calculations
At the end of the 1st page there is a + instead of a - inside the square root. So is it in the 2 first equations in the second page
Then you made an error in the second equation of the second page in the simplification by
The right equation is
But I agree with you : the disappearance of is strange