the school year is divided into two semesters the first of which is september through december ,,and the secnd january through may .这个定语从句那部分是定语哪部分是先行词并翻译一下,

先行词：two semesters
定语：which is september through december...
翻译：一学年分为两个学期，第一个学期是九月到十二月，第二个是一月到五月
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一学年分为2个学期，第一个学期从9月到12月，第二个学期从1月到3月。
which修饰two semesters（先行词貌似），second后面省略of which is
即the school year is divided into two semesters; the first of the two semesters is september through december, ,and the secnd of the two semesters january through may.

一学年分为2个学期,第一个学期从9月到12月,第二个学期从1月到3月.
先行词：semesters
定语从句：the first of which is september through december ,and the secnd january through may

WHICH前面是先行词，后面的是定语。在学校的时间被分为了两个学期，第一学期是从九月到十二月，第二学期是从一月到五月