8
a1+a2+a3=3a2=15
a2=5=a1+d
d=3
a4+a5+a6=3a1+12d=6+36=42
9
a2=a1+d=1
s5=5a1+10d=10
a1=0
d=1
s7=7a1+21d=21
10
a1+a6=a3+a4=1
所以S6=6(a1+a6)/2=3*1=3
12
an=n*a(n+1)-nan
(n+1)an=n*an+1
an+1/an=(n+1)/n
所以
an/an-1=n/(n-1)
an=an/an-1*(an-1/an-2)...(a2/a1)=an/a1=n