3 sentences

1. 几何学研究平面和立体的性质。

   jǐ​hé​xué yán​jiū píng​miàn hé lì​tǐ de xìng​zhì.

   Geometry studies the properties of planes and solids.

   Source: Dong Chinese Dictionary

2. 圆是平面上到定点距离等于定长的点的轨迹。

   yuán shì píng​miàn shàng dào dìng​diǎn jù​lí děng​yú dìng​cháng de diǎn de guǐ​jì.

   A circle is the locus of points in a plane that are at a fixed distance from a fixed point.

   Source: Dong Chinese Dictionary

3. 这种产品目前只在平面媒体上进行宣传。

   zhè zhǒng chǎn​pǐn mù​qián zhǐ zài píng​miàn méi​tǐ shàng jìn​xíng xuān​chuán.

   This product is currently only being advertised in print media.

   Source: Dong Chinese Dictionary