奇数项是n
偶数项是n-1
则[a1+a(2n-1)]n/2=36
a1+a(2n-1)=72/n
[a2+a(2n-2)]*(n-1)/2=30
a2+a(2n-2)=60/(n-1)
等差则a1+a(2n-1)=a2+a(2n-2)
所以72/n=60/(n-1)
60n=72n-72
n=6