[tex]\displaystyle\bf\\1)\\\\a_{1} =3\\\\a_{2} =1\\\\a_{18}=?\\\\\\a_{2} =a_{1} +d\\\\d=a_{2} - a_{1} =1-3=-2\\\\a_{18} =a_{1}+17d=3+17\cdot(-2)=3-34=-31\\\\Otvet \ : \ a_{18} =-31\\\\\\2)\\\\d=0,5\\\\a_{9} =3\\\\a_{1} =?\\\\\\a_{9} =a_{1} +8d\\\\a_{1} =a_{9} -8d=3-8\cdot 0,5=3-4=-1\\\\Otvet \ : \ a_{1} =-1[/tex]