[tex]a_{12} = 30\\S_{12} = 162\\n = 12\\a_1 = ?\\\\S_{n} = \frac{a_1+a_{n}}{2}\cdot n\\\\a_{n} = a_{12} = 30\\\\S_{12} = \frac{a_1 + a_{12}}{2}\cdot n\\\\162 = \frac{a_1+30}{2}\cdot 12\\\\162 = (a_1+30)\cdot6 \ \ /:6\\\\27 = a_1 + 30\\\\a_1 = 27-30\\\\a_1 = -3\\----[/tex]
