JEE Advance - Mathematics (1985 - No. 20)

Three lines $$px + qy + r = 0$$, $$qx + ry + p = 0$$ and $$rx + py + q = 0$$ are concurrent if

$$p + q + r = 0$$

$${p^2} + {q^2} + {r^2} = qr + rp + pq$$