Sn-S6=A7+A8+……+A(n-1)+An=324-36=288
S(n-6)=A1+A2+……+A(n-7)+A(n-6)=144
(Sn-S6)-S(n-6)=(A7-A1)+(A8-A2)+……+(A(n-7)-A(n-1))+(An-A(n-6))
=6d+6d+……+6d+6d
=(n-6)6d=288-144=144
A4+A5+……+A(n-4)+A(n-3)
=(A1+3d)+(A2+3d)+……+(A(n-7)+3d)+(A(n-6)+3d)
=(A1+A2+……+A(n-7)+A(n-6))+(n-6)3d
=144+72=216
A4+A5+……+A(n-4)+A(n-3)
=(A4+A(n-3))(n-6)/2=216
A4+A(n-3)=A1+3d+A1+(n-4)d=A1+A1+(n-1)d=A1+An
(A1+An)(n-6)/2=216
Sn=(A1+An)n/2=324
两式相除
(n-6)/n=216/324
n=18