Class 11, NCERT book,Chaapter 3 , exercise 3.3, Question no. 18I have

This question was asked in Ex 3.3, 18

Class 11, NCERT book,
Chaapter 3 , exercise 3.3, Question no. 18

I have a doubt in this question that why (x-y)/2 is not cancelled in the last whereas (x y)/2 is cancelled.