1=0? + 1=( 1- 1)? + 1

2= 1? + 1=(2- 1)? + 1

5=2? + 1=(3- 1)? + 1

10=3? + 1=(4- 1)? + 1

…………

Rule: Starting from the item 1, each item is equal to the square of the difference between the items minus one, plus 1.

Ann =(n- 1)? + 1=n？ -2n+ 1+ 1=n？ -2n+2

The general formula of the sequence {an} is an=n? -2n+2