Question

fie sirul 3,9,15,21.....:a)completati sirul cu inca doi termeni b)determinati al 100-lea termen al sirului c)aflati suma primilor 50 de termeni

Answer 1

a) 3,9,15,21, 27,33,39,45,51,57,......  (a₂-a₁=9-3=6=> r = 6)

b) a₁₀₀ = a₁+99r = 3+99x6 = 3+594 = 597

c) a₅₀ = a₁+49r = 3+49x6 = 3+294 = 297

S₅₀ = 3+9+15+21+.....+297 = (3+297)x50:2 = 300x25 = 7500