3 sentences for 其一 (qí​yī — one of)

1. 第见其一，不见其二。

   dì jiàn qí​yī, bù jiàn qí'èr.

   One sees only one aspect while ignoring the other.

   Source: Dong Chinese Dictionary

2. 这是导致失败的原因其一。

   zhè shì dǎo​zhì shī​bài de yuán​yīn qí​yī.

   This is one of the reasons for the failure.

   Source: Dong Chinese Dictionary

3. 他有两子，其一在京，其二在沪。

   tā yǒu liǎng zǐ, qí​yī zài Jīng, qí'èr zài Hù.

   He has two sons; one is in Beijing, and the other is in Shanghai.

   Source: Dong Chinese Dictionary