1+4+7+...+x=1717
Avem: an= x, a₁=1, r=3
x=a₁+(n-1)*r
x=1+(n-1)*3
x=1+3n-3⇒x=3n-2
S=(a₁+x)*n /2
(1+3n-2)*n/2= 1717
(3n-1)*n=3434
3n²-n-3434=0
Δ=1+4*3*3434
Δ=41209⇒√Δ=203
n=1+203/6⇒n=34
Am considerat doar valoarea pozitiva,(nu are rost sa mai calculam si cealalta solutie a ecuatiei).
x=3n-2
x=100
Hope you really understand.
