Question

найдите тринадцатый член арифметической прогрессии -12;-9,5​

Answer 1

$\displaystyle\bf\\a_{1} =-12\\\\a_{2} =-9,5\\\\\\a_{2} =a_{1} +d\\\\d=a_{2} - a_{1} =-9,5-(-12)=-9,5+12=2,5\\\\a_{13} =a_{1} +12d=-12+12\cdot 2,5=-12+30=18\\\\Otvet \ : \ a_{13} =18$