a1²+a10²=10
S=a11+a12+.+a18
=(a1+10d)+(a1+11d)+.+(a1+17d)
=8a1+108d
a1=(S-108d)/8
a10=a1+9d=(S-36d)/8
a1²+a10²=10
[(S-108d)/8]²+[(S-36d)/8]²=10
(S-108d)²+(S-36d)²=640
2S²-288dS+12960d²=640
6480d²-144Sd+S²-320=0
△=(144S)²-4*6480(S²-320)≥0
5184S²-6480(S²-320)≥0
-S²+1600≥0
S²≤1600
-40≤S≤40