Question

6 будь ласка!!!!!!!!!!!!!!!!

Answer 1

$\displaystyle\bf\\\left \{ {{a_{12} =-47} \atop {a_{7} =-27}} \right. \\\\\\-\left \{ {{a_{1} +11d=-47} \atop {a_{1} +6d=-27}} \right. \\----------\\5d=-20\\\\d=-4\\\\a_{1} =-27-6d=-27-6\cdot(-4)=-27+24=-3\\\\\\S_{15} =\frac{2a_{1}+14d }{2} \cdot 15=\frac{2\cdot(-3)+14\cdot(-4)}{2} \cdot 15=\frac{-6-56}{2} \cdot 15=\\\\\\=\frac{-62}{2} \cdot 15=-31\cdot 15=-465\\\\\\Otvet: \ S_{15} =-465$