LU分解矩阵：
| 2 2 |
| 4 9 |如果U的对角线元素都是1，那么L的下对角线元素l22是（ ）
4
5
6
7
LU分解（其中“LU”代表“上下”，也称为LU分解）将矩阵分解为下三角矩阵和上三角矩阵的乘积。
| 2 2 | = | l11 0 | * | 1 u12 |
| 4 9 | | l21 l22 | | 0 1 |
l21 * u12 + l22 * 1 = 9 ------ (1)
We need to find l21 and u12
l21 * 1 + l22 * 0 = 4
l21 = 4
l11 * U12 + 0 * 1 = 2
l11 = 2
U12 = 1
Putting value of l21 and u12 in (1), we get
4 * 1 + l22 * 1 = 9
l22 = 5
| 2 2 | = | l11 0 | * | 1 u12 |
| 4 9 | | l21 l22 | | 0 1 |
l21 * u12 + l22 * 1 = 9 ------ (1)
We need to find l21 and u12
l21 * 1 + l22 * 0 = 4
l21 = 4
l11 * U12 + 0 * 1 = 2
l11 = 2
U12 = 1
Putting value of l21 and u12 in (1), we get
4 * 1 + l22 * 1 = 9
l22 = 5
