Răspuns:
a)1+4+7+....+x=117
2+((n-1) ×3)/ ×n=117
(-1+3n) ×n=234
3n²-n-234=0
Δ=532
N=9
x=a1+8r=25
x=25
b) 1+8+15+....+x=3075
1+(1+7)+(1+7×2)+....+x=3075
x=1+7n
1+(1+7)+(1+7×2)+....+(1+7n)=3075
1×(n+1)+7(1+2+....+n)=3075
n+1+7×n×(n+1)/2=3075
2n+2+7n²+7n=6150
7n²+9n-6148=0
7n²-203n+212n-6148=0
7n(n-29)+212(n-29)=0
(n-29)(7n+212)=0
n=29
x=1+7×29=204
x = 204
