un=∣∣
∣
∣
∣
∣
∣
∣∣a1100......0−1a210......00−1a31......0.....................00......−1an−1100......0−1an∣∣
∣
∣
∣
∣
∣
∣∣
Expand with respect to the bottom row. Such determinants are called containants
=−(−1)∣∣
∣
∣
∣
∣∣a1100......0−1a210......00−1a31......0.....................00......−1an−11∣∣
∣
∣
∣
∣∣+an∣∣
∣
∣
∣
∣∣a1100......0−1a210......00−1a31......0.....................00.........−1an−11∣∣
∣
∣
∣
∣∣ =un−2+anun−1
Hence, un=anun−1+un−2