[tex]a_n=\sqrt{2n^2+1}\\\\\\a_1=\sqrt{2\cdot1^2+1}=\sqrt3\\\\a_2=\sqrt{2\cdot2^2+1}=\sqrt9=3\\\\a_3=\sqrt{2\cdot3^2+1}=\sqrt{19}\\\\a_4=\sqrt{2\cdot4^2+1}=\sqrt{33}\\\\a_{12}=\sqrt{2\cdot12^2+1}=\sqrt{289}=17[/tex]
