[lnพwsะบุ๑sสุ๑xล่o]

11.$f(f(f(-\frac{1}{3} )))=f(f(-3))=f(\frac{1}{6} )=6$

18.$a_n=\frac{1}{4+8+12+...+4n}= \frac{1}{2n(n+1)}=0.5(\frac{1}{n}-\frac{1}{n+1})$

$a_1+a_2+...=0.5(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...)=0.5$