# Linear Algebra

#### 方程组的几何解释

x + 2y = 13;
3x - y = 4;


  -   -     -    -   -    -
| 1 |     |  2 |   | 13 |
x |   | + y |    | = |    |
| 3 |     | -1 |   |  5 |
-   -     -    -   -    -


-      -   -   -   -    -
| 1  2 |   | x |   | 13 |
|      | * |   | = |    |
| 3 -1 |   | y |   |  5 |
-      -   -   -   -    -

When $a \ne 0$, there are two solutions to $$ax^2 + bx + c = 0$$ and they are $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$ $\begin\left\{bmatrix\right\} 3 & 5\\ 1 & 2 \end\left\{bmatrix\right\} \cdot \begin\left\{bmatrix\right\} x_1 \\ x_2 \end\left\{bmatrix\right\} = \begin\left\{bmatrix\right\} 4 \\ 1 \end\left\{bmatrix\right\}$