=2l2+2m2+2lm+2n2+2mn+2nl [After re-arranging]
OR,
l2+m2+m2+n2+n2+l2+2lm+2mn+2nl
=[l2+m2+2mn]+[m2+n2+2mn]+[n2+l2+2nl]
[Applying the identity (a2+b2+2ab)=(a+b)2]
=(l+m)2+(m+n)2+(n+l)2
Add the following.
(i) ab − bc, bc − ca, ca − ab
(ii) a − b + ab, b − c + bc, c − a + ac
(iii) 2p2q2 − 3pq + 4, 5 + 7pq − 3p2q2
(iv) l2 + m2, m2 + n2, n2 + l2, 2lm + 2mn + 2nl