11, 16, 21 , 26... is arithmetic sequence ,where
a(1)=11,
d=16-11=21-16=5
Sum of arithmetic sequence
Sn=(a(1)+a(n))*n/2
a(n)=a(1)+d(n-1)
S(n)=(a(1)+a(1)+d(n-1))n/2
S(n)=(2a(1)+d(n-1))*n/2
1170=(2*11+5(n-1))*n/2
2*1170=22n+5n²-5n
2*1170=17n+5n²
5n²+17n-2340=0
D=b²-4ac=17²+4*5*2340=47089
n=(-b+/-√D)/2a
n=(-17+/-√47089)10
n=(-17+/-217)/10
We take only positive root, because number of row can be only positive.
n=200/10=20 row
