progresie atmetica
a₁ = 8; r = 7 ==> an = a₁ + ( n - 1) x r
8 + ( n - 1 ) x 7 = 1 513
( n - 1 ) x 7 = 1 513 - 8 = 1 505
n - 1 = 1 505 : 7
n - 1 = 215
n = 215 + 1 = 216 → pozitia pe care se gaseste numarul 1 513
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b)
S = 8 + 15 + 22 + 28 + ....... + 1513
→ aflam cati termeni are suma
( 1 513 - 8 ) : 7 + 1 = 1 505 : 7 + 1 = 215 + 1 = 216 termeni are suma
S = 216 x ( 8 + 1 513 ) / 2
S = 108 x 1 521
S = 164 268
