Add following: (i) ab−bc, bc−ca, ca−ab (ii) a−b+ab, b−c+bc, c−a+ac (iii) 2p²q²−pq+4, 5+7pq−p²q² (iv) l²+m², m²+n², n²+l², 2lm+2mn+2nl

(i) Given (ab−bc) + (bc−ca) + (ca−ab) = ab−bc + bc−ca + ca−ab
Grouping the like terms as = ab−ab + bc−bc + ca−ca
All terms gets cancelled, So = 0
Hence, the addition of given expression is 0
(ii) Given (a−b+ab) + (b−c+bc) + (c−a+ac) = a−b+ab+b−c+bc+c−a+ac
Grouping the like terms as = a−a + b−b + c−c + ab+bc+ca
By simplifying, we get = ab+bc+ac
(iii) Given (2p²q²−pq+4) + (5+7pq−p²q²) = 2p²q²−pq+4+5+7pq−p²q²
Grouping the like terms, we get = 2p²q²−p²q² + 7pq−pq + 5+4
By simplifying, we get = p²q² + 6pq + 9
(iv) Given (l²+m²) + (m²+n²) + (n²+l²) + (2lm+2mn+2nl)
Grouping the like terms, we get = l²+l² + m²+m² + n²+n² + 2lm+2mn+2nl
= 2l²+2m²+2n² + 2(lm+mn+nl)
By simplifying, we get = 2(l²+m²+n²+lm+mn+nl)