∣∣a−bca+1b+1c−1a−1b−1c+1∣∣+(−1)n∣∣a+1a−1ab+1b−1−bc−1c+1c∣∣=∣∣a−bca+1b+1c−1a−1b−1c+1∣∣+(−1)n∣∣a+1b+1c−1a−1b−1c+1a−bc∣∣ =∣∣a−bca+1b+1c−1a−1b−1c+1∣∣+(−1)n+1∣∣a+1b−1c−1a−bca−1b−1c+1∣∣=∣∣a−bca+1b+1c−1a−1b−1c+1∣∣+(−1)n+2∣∣a−bca+1b+1c−1a−1b−1c+1∣∣
This is equal to zero only if n + 2 is odd i.e. n is odd integer.