31. 如图,在三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中,过A1B1A_{1}B_{1}A1B1的截面与ACACAC交于点DDD,与BCBCBC交于点E(DE(DE(D,EEE都不与CCC重合),若该截面将三棱柱分成体积之比为2:12:12:1的两部分,则CDAC=(\frac{CD}{AC}=(ACCD=( )))