Δs=aT^2所以：s4-s1=3a1T^2s5-s2=3a2T^2s6-s3=3a3T^2a=(a1+a2+a3)/3 =[(s4-s1)/(3T^2)+(s5-s2)/(3T^2)+(s6-s3)/(3T^2)]/3 =(s4+s5+s6-s1-s2-s3)/(9T^2)
