S 17= 2a1+16d / 2 * 17
a9 = a1+ 8d
2*2=4
4/2=2
2*17= 34
4/2*17
s19
s19= 2a1+18d/2 * 19
a10= a1 + 9d
2(a1+9d)/2*19
4*19
S2001 = 2a1+2000d/ 2 * 2001
a1001= 2000
a1 +1000d = 2000
2(a1+1000d)/2 * 2001= 2000*20001= 4002000
a5=11
a2=5
5-2=3
11-5/3=2
a6-a4=8
6-4=2
8:2=4
a10+a12=8
a11=?