解如下图所示
道理：an=a1+(n-1)d，a1+an=a1+a1+(n-1)d=2a1+(n-1)d，a1+a12=2a1+11da6+a7=a1+5d+a1+6d=2a1+11d=a1+a12=12Sn=n(a1+an)/2，S12=12(a1+a12)/2=6(a1+a12)=6×12=72
