24. The general term formula of the sequence $1,2,3,1,2,3, \cdots$ is  
$a_{n}=$ $\qquad$ , and the sum of the first $n$ terms $S_{n}=$ $\qquad$ . (Express each using a single formula)
$24.2+\frac{2}{\sqrt{3}} \sin \frac{2(n-2) \pi}{3}, 2 n-\frac{4}{3} \sin ^{2} \frac{n \pi}{3}$ Observing, we know that this sequence is a periodic sequence with a period of 3.