Numarul de termeni este:
[tex]n = \frac{58-1}{3}+1 = \frac{57}{3}+1 = 19+1 = 20\;de\;termeni.[/tex]
(3x - 1) + (3x - 4) + (3x - 7) +...+ (3x - 58)=790
(3x + 3x + 3x +.......de 20 de ori.....+ 3x ) - (1 + 4 + 7 + .... + 58) = 790
20 ∙ 3x - (20 ∙ 59 / 2 ) = 790
60x - (10 ∙ 59 )=790
60x - 590 = 790
60x = 790 + 590
60x = 1380
x = 1380 : 60 = 23
x = 23
