Răspuns:
Explicație pas cu pas:
a) Sₙ = 1+3+5+....+(2n+1)
Sₙ = (1+2n+1) ·[(2n+1-1):2+1]:2
Sₙ = (2n+2)·(n) : 2
Sₙ = n(n+1)
b) Sₙ = 3+7+11+.....+(4n-1)
Sₙ = (4n-1+3)·[(4n-1-3):4 +1]:2
Sₙ = (4n+2)·n:2 =>
Sₙ = n·(2n+1)
c) Sₙ = 2x+(2x-3) + (2x-6) + ....+ (2x-3n+3)
Sₙ = (2x+2x-3n+3)·[(2x-3n+3-2x):(-3)+1]:2
Sₙ = (4x-3n+3)·n:2
