Расчет: XhzMnO - 1

S__n =

(a_₁+a__n)* n

/ 2

S__n =

(a_₁+a__n)* n

/ 2

$S_{n}$ = $\frac{(a_{1}+a_{n})\cdot n}{2}$

S__n =

a_₁* n

/ 2

+

a__n* n

/ 2

$S_{n}$ = $\frac{a_{1}\cdot n}{2}+\frac{a_{n}\cdot n}{2}$

S__n = 0.5* a_₁* n+ 0.5* a__n* n $S_{n}$ = $0.5\cdot a_{1}\cdot n+0.5\cdot a_{n}\cdot n$