写出通项公式1/2,4/3,9/4,16/51,0,-1,07,77,777,7777-1+1/2,3-1/4,-5+1

①
AN = N²/(N+1)
②
AN = (-1)^(N+1) /2 + 1/2
③
AN = 7 *(10^N - 1)/9
④
AN = (-1)^N * [(2N-1) - (1/2)^N]