5) (2a^2b-b )/ a - ab = (2a^2b-b )/ a - a^2b / a = (2a^2b -b -a^2b)/ a = (a^2b -b) / a
6) a -b/x -a/x^2 = (ax^2 ) /x^2 -(bx)/x^2 -a / x^2 = (ax^2 -bx-a) / x^2
7) 5 -1/x -1/y = 5xy/xy - y/xy - x/xy = (5xy -y -x) / xy
8)a -(a-1)/2 +(a-2)/3 = 6a/6 - 3(a-1)/6 +2(a-2)/6 = (6 -3a+1+2a-4)/6 = (3-a)/6