Add:
(i) 3mn , - 5 mn , 8 mn - 4mn
(ii) t - 8tz , 3tz - z , z - t
(iii) -7 mn + 5,12 mn + 2, 9 mn - 8, - 2 mn - 3
(iv) a + b - 3, b - a + 3, a - b + 3
(v) 14 x + 10 y - 12 xy - 13,18 - 7 x - 10 y + 8 xy , 4xy
(vi) 5 m - 7 n , 3n - 4 m = 2, 2 m - 3mn - 5
(vii) 4 x2 y , -3 xy2 , -5 xy2 , 5x2 y
(viii) 3 p2 q2 - 4 pq + 5, - 10 p2 q2 ,15 + 9 pq + 7 p2 q2
(ix) ab - 4 a , 4b - ab , 4 a - 4b
(x) x2 - y2 - 1, y2 -1 - x2 ,1 - x2 - y2