Add the following.
(i) `ab-bc, bc-ca,ca-ab`
(ii) `a-b+ab,b-c+bc,c-a+ac`
(iii) `2p^2q^2-3pq+4,5+7pq-3p^2q^2`
(iv) `l^2+m^2,m^2+n^2,n^2+l^2,2lm+2mn+2nl`

Explanation

(i) `ab-bc+bc-ca+ca-ab`

`=ab-ab+bc-bc+ca-ca`

`=0`

(ii) `a-b+ab+b-c+bc+c-a+ac`

`=a-a+b-b+c-c+ab+bc+ca`

`=ab+bc+ca`

(iii) `2p^2q^2-3pq+4+5+7pq-3p^2q^2`

`=2p^2q^2-3p^2q^2+7pq-3pq+5+4`

...