等差数列{an}
s4,s8-s4,s12-s8也成等差数列
2(s8-s4)=s4+s12-s8
2s8-2s4=s4+s12-s8
s12=3s8-3s4
s12=3(s8-s4)
s12=3*(4-8)
s12=3*(-4)
s12=-122(s8-s4)=s4+s12-s8 这部是怎么算的？s4,s8-s4,s12-s8也成等差数列s8-s4是s4和s12-s8的等差中项