Question

写出数列通项公式：2/3,-4/9,2/9,-8/81.

Answer 1

2/3x(-2/3)=-4/9 S2/S1=-2/3

-4/9x(3/6)=2/9 S3/S2=3/6

2/9x(-4/9)=-8/81 S4/S3=-4/9

括号中分子为-2、3、-4、分母为3、6、9

故数列通项公式是Sn=(-1)n(n+1)/3nSn-1