shù / xué / kè / shàng / lǎo / shī / jiè / shào / le / xī / là / zì / mǔ / xī / gé / mǎ / de / liǎng / zhǒng / xiě / fǎ

数学课上，老师介绍了希腊字母西格玛的两种写法。

In math class, the teacher introduced the two written forms of the Greek letter sigma.