0

0

S9=(a1+a9)*9/2=27,a1+a9=6
S29=(a1+29)*29/2=290,a1+a29=20
2a5=a1+a9=6,a5=3
2a15=a1+a29=20,a15=10
a1+a19=a5+a15=13
그러므로 S19=(a1+a19)*19/2=13*19/2=247/2