[tex]\displaystyle 1,7,13,19 \\ a_1=1,a_2=7,a_3=13,a_4=19 \\ r=7-1 \Rightarrow r=6 \\ a_{10}=a_{10-1}+r \Rightarrow a_{10}=a_9+r \Rightarrow a_{10}=a_1+9r \Rightarrow a_{10}=1+9 \cdot 6 \Rightarrow \\ \Rightarrow a_{10}=1+54 \Rightarrow \boxed{a_{10}=55} [/tex]
