[tex]\displaystyle\bf\\a_{1} + a_{2} =8,8\\\\a_{1} -d=1,7\\\\\\1)\\\\\left \{ {{a_{1} +a_{1} +d=8,8} \atop {a_{1} -d=1,7}} \right. \\\\\\+\left \{ {{2a_{1} +d=8,8} \atop {a_{1} -d=1,7}} \right. \\----------\\3a_{1} =10,5\\\\a_{1} =10,5:3=3,5\\\\d=a_{1}-1,7=3,5-1,7=1,8\\\\a_{3} =a_{1} +2d=3,5+2\cdot 1,8=3,5+3,6=7,1\\\\\boxed{a_{3} =7,1}\\\\\\2)\\\\a_{n} =32,3 \ \ \ , \ \ \ n=?\\\\a_{n} =a_{1} +d\cdot(n-1)\\\\32,3=3,5+1,8\cdot(n-1)\\\\32,3=3,5+1,8n-1,8\\\\32,3=1,8n+1,7[/tex]

[tex]\displaystyle\bf\\1,8n=32,3-1,7\\\\1,8n=30,6\\\\n=30,6:1,8\\\\\boxed{n=17}\\\\Otvet \ : \ a_{17} =32,3[/tex]