Question

Qual a soma dos multiplos de 3 compreendidos entre 11 e 780?

Answer 1

a1 = 12; an = 780; r = 3; n = ?; an = a1+(n-1).r; 780 = 12+(n-1).3; 780 = 12+3n-3; 780= 9+3n; 780-9 = 3n; 771= 3n; 3n = 771; n= 771/3, n = 257; Sn= (a1+an).n/2; S257 = (12+ 780).257/2; Sn= 792.257/2; SN = 396.257; SN= 101772; Resp.: S257 = 101772