👤

scrieti suma primilor 20 de termeni a unei progresii aritmetice, daca: a1=2; r= 3

Răspuns :

Sn = [2a1 + (n-1)r] × n/2
S20 = [2×2 + (20 - 1)×3]×20/2 = (4 + 19×3)×10= (4 + 57)×10 = 61×10 = 610