3的负一次方加3的负二次方加到3的负三次方该怎么算

2个回答

1/3+1/3^2+1/3^3+.+1/3^8
=(1/3(1+1/3^2)+1/3^2(1+1/3^2))(1+1/3^4)
=4/9*10/9*82/81
=3280/6561