[tex]Se~observa~ca~sirul~este~periodic: \\ \\ a_1=4~;~a_2=8~;~a_3=2~;~a_4= \frac{1}{4}~;~ a_5=\frac{1}{8}~;~a_6= \frac{1}{2} ~;\\ \\ a_7=4~;~a_8=8~:~............ \\ \\ Perioada~are~lungimea~de~6. \\ \\ S=(a_1+...+a_6)+(a_7+...+a_{12})+...+(a_{1999}+...+a_{2004})= \\ \\ =334(4+8+2+ \frac{1}{4}+ \frac{1}{8}+ \frac{1}{2})=...~.[/tex]
