Question

sa se determine suma primilor n termeni ai progresiei aritmetice an daca
a1= -1, r=3,n=12

Answer 1

$S_n=\frac{2a_1+(n-1)*r}{2}*n \\\\ S_n=\frac{-2+(12-1)*3}{\not2}*\not12\\\\ S_n=(-2+11*3)*6 \\\\ S_n=(-2+33)*6 \\\\ S_n=31*6 \\\\ \boxed{S_n=186}$