a1=4
a2=10
r=a2-a1=10-4=6
an=a1+(n-1)r
Sn=(a1+an)*n/2
2Sn=(a1+a1+(n-1)r)*n
2Sn=(2a1+(n-1)r)*n
2*3104=(2*4+(n-1)*6)*n
6208=(8+6n-6)*n
6208=(2+6n)*n
6n^2+2n-6208=0
3n^2+n-3104=0
Δ=1+4*3*3104=37249
√Δ=193
n∈N, deci ne trebuie doar valoarea pozitiva
n=(-1+193)/6=192/6=32
x=an=a1+(n-1)r=4+31*6=190
