Question

SA SE DETERMINE SUMA PRIMILOR n TERMENI AI PROGRESIEI ARITMETICE
An ( a indice n ) daca :
a1= -1
r = 3
n=12

Answer 1

$\displaystyle a_1=-1~,~r=3~,~n=12 \\ \\ \boxed{S_n= \frac{2a_1+(n-1) \cdot r}{2}\cdot n } \\ \\ S_n= \frac{2 \cdot (-1)+(12-1) \cdot 3}{2} \cdot 12 \\ \\ S_n= \frac{-2+11 \cdot 3}{\not2} \cdot \not12 \\ \\ S_n=(-2+33) \cdot 6 \\ \\ S_{n}=31 \cdot 6$

$S_n=186$