8
S20=20a1+190d
S40=40a1+780d
S20=S40
20a1=-590d
20a1+590d=0
(10a1+290d)+(10a1+300d)=0
10a30+10a31=0
10a1+10a60=0
a1+a60=0
60(a1+a60)/2=0
S60=0
9
S12=21
12a1+66d=0
a2+a5+a8+a11
=4a1+(d+4d+7d+10d)
=4a1+22d
=(1/3)(12a1+66d)
=(1/3)S21
=7
10
S9=72
9a1+36d=72
a2+a4+a9
=3a1+(d+3d+8d)
=3a1+12d
=(1/3)(9a1+36d)
=(1/3)S9
=24
11
S9=9(a1+a9)/2=9a5
S5=5(a1+a5)/2=5a3
S9/S5=(9/5)*(a5/a3)=9
12
6S5-5S3
=6(5a1+10d)-5(3a1+3d)
=15a1+45d
=15(a1+3d)
=15a4
a4=(1/15)(6S5-5S3)
=1/3