Răspuns:
Explicație pas cu pas:
x+2x+3x+....+(n+1)x = n+(n+1)+(n+2)+....+(n+n) <=>
x(1+2+3+....+n +n+1) = n·(n+1) + (1+2+3+....+n) <=>
x(1+n+1)·(n+1):2 = n(n+1) + (1+n)·n:2 <=>
x(n+2)(n+1):2 = n(n+1)(1+n):2 =>
x(n+2) = n(n+1) =>
x = n(n+1)/(n+2) ; x ; n ∈ N =>
n = 0 => x = 0
#copaceibrainly
