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 x^{2} y , -3 xy^{2} , -5 xy^{2} , 5x^{2} y

(viii) 3 p^{2} q^{2 }- 4 pq + 5, - 10 p^{2} q^{2} ,15 + 9 pq + 7 p^{2 }q^{2}

(ix) ab - 4 a , 4b - ab , 4 a - 4b

(x) x^{2} - y^{2} - 1, y^{2} -1 - x^{2} ,1 - x^{2} - y^{2}