Question

Знайти суму двадцяти перших членів арифметичної прогресії: 3; -2; -7; ...

Answer 1

$\displaystyle\bf\\a_{1} =3\\\\a_{2} =-2\\\\S_{20} =?\\\\\\\\a_{2} =a_{1} +d\\\\d=a_{2} - a_{1} =-2-3=-5\\\\a_{20} =a_{1} +19d=3+19\cdot(-5)=3-95=-92\\\\\\S_{20} =\frac{a_{1} + a_{20} }{2} \cdot 20=(a_{1} + a_{20} )\cdot 10=(3-92)\cdot 10=-89\cdot 10=-890$