\(\frac{a1}{a2}=\frac{a2}{a3}=\frac{a3}{a4}=...=\frac{a9}{a1}\)
\(\Rightarrow\frac{a1}{a2}=\frac{a2}{a3}=\frac{a3}{a4}=...=\frac{a9}{a1}=\frac{a1+a2+a3+...+a9}{a2+a3+a4+...+a1}=1\)
\(\Rightarrow a1=a2,a2=a3,a3=a4,...,a9=a1\)
\(\Rightarrow a1=a2=a3=a4=a5=...=a9\)
Mà a1=5\(\Rightarrow a5=5\)