ak1,ak2,ak3.构成等比^2=ak1*ak3
(a1+d)^2=(a1)*（a1+(6-1)d） 计算d=3a1
ak4=^2/ak2=a1+(k4-1)*d d为方差
(a1+5*d)^2/(a1+d)=a1+(k4-1)*d 代入d=3a1
即可计算得K4=22