Question

Знайти суму семи перших членів арифметичної прогресії -13; -12; -11

Answer 1

$a_{1}=-13\\\\a_{2}=-12\\\\a_{2}=a_{1}+d\\\\d=a_{2}-a_{1}=-12-(-13)=-12+13=1\\\\S_{7}=\frac{2a_{1}+6d}{2} *7=(a_{1}+3d)*7=(-13+3*1)*7=-10*7=-70\\\\\boxed{S_{7}=-70}$

Answer 2

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