a1=1,r=4 si Sn=231(progresie aritmetica)
Sn=[2xa1+(n-1)xr]xn/2
231=[2x1+(n-1)x4]xn/2
231=(2+4n-4)xn/2
231=2n+4n^2-4n/2
231=4n^2-2n/2
231=2(2n^2-n)/2
231-2n^2-n
2n^2-n-231=0
de aici rezolvam o ec de gr 2(cu delta)
Δ=b^2-4ac
Δ=1849
n1=-b+√Δ/2a
n1=1+43/4=-42/4=-21/2
n2=-b-√Δ/2a
n2=1+43/4=11
n1 nu convine
n2=11-progresia are 11 termeni
a1=1,a2=5,a3=9
a4=a3+r=9+4=13
a5=13+4=17
a6=17+4=21
a7=25,a8=29,a9=33,a10=37,a11=41
a11=x=41
