Расчет: BCfePX - 1

S__n =

( 2* a_₁+ d* (n-1))* n

/ 2

S__n =

( 2* a_₁+ d* (n-1))* n

/ 2

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

S__n = a_₁* n+

n^²* d

/ 2

-

d* n

/ 2

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

S__n = a_₁* n+ 0.5* d* n^²- 0.5* d* n $S_{n}$ = $a_{1}\cdot n+0.5\cdot d\cdot n^{2}-0.5\cdot d\cdot n$