第三讲：乘法和逆矩阵 用例计算失误

**您好！在学习过程中发现《第三讲：乘法和逆矩阵》有一处用例计算失误，具体位置在“逆（方阵）”一节第7行，原文如下：**
> **另一种判定方法，如果存在非零向量，使得，则矩阵不可逆。我们来用上面的矩阵为例：。**

**应改为。**
### 在此非常感谢您的付出！

Locate the Lesson 3 section “Inverse (square matrix)” and inspect the seventh line in the example. Verify the corrected nonzero vector multiplication shown in the issue, then update the lesson text and confirm the displayed calculation is mathematically consistent.

Written by the indexing model from the issue text.