5 sentences

1. 我的刀变钝了。

   wǒ​de dāo biàn dùn le.

   My knife has become dull.

   Source: Tatoeba

2. 一把钝刀并不是一把好刀，因为刀的目的是切开。

   yī​bǎ dùn dāo bìng​bù​shì yī​bǎ hǎo dāo, yīn​wèi dāo de mù​dì shì qiē kāi.

   A dull knife is not a good knife because the purpose of a knife is to cut.

   Source: CCMatrix

3. 我学得比较迟钝。

   wǒ xué de bǐ​jiào chí​dùn.

   I'm a slow learner.

   Source: OpenSubtitles

4. 互联网令我们更聪明还是更愚钝？

   hù​lián​wǎng lìng wǒ​men gèng cōng​ming hái​shi gèng yú​dùn?

   Is the internet making us smarter or dumber?

   Source: CCMatrix

5. 所以，一个三角形不可能有两个钝角。

   suǒ​yǐ, yī​ge sān​jiǎo​xíng bù​kě​néng yǒu liǎng​ge dùn​jiǎo.

   Therefore, a triangle can’t have two obtuse angles.

   Source: CCMatrix