tri·go·nal /traɪˈgonḷ/
  (a.)三角的,三角形的,斷面呈三角的
  Trig·o·nal a. Having three angles, or corners; triangular; as, a trigonal stem, one having tree prominent longitudinal angles.
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  trigonal
       adj : having three-fold symmetry [syn: rhombohedral]