Given: p(p2+q2−r2)+q(r2−q2−p2)−r(p2+q2−r2)
=p(p2+q2−r2)−q(−r2+q2+p2)−r(p2+q2−r2)
=p(p2+q2−r2)−q(p2+q2−r2)−r(p2+q2−r2)
The common factor is (p2+q2−r2)
Dividing by (p2+q2−r2) in above expression ,we get,
p(p2+q2−r2)(p2+q2−r2)−q(p2+q2−r2)(p2+q2−r2)−r(p2+q2−r2)(p2+q2−r2)
=p−q−r
∴p(p2+q2−r2)+q(r2−q2−p2)−r(p2+q2−r2)=(p2+q2−r2)(p−q−r)