% 6.6: special types of matrices: 3(c)、7(b)、 (LDL^t)
6.6: special types of matrices: 17、19(too easy)、20、21,
    new: 25

7.3 Iterattive techniques: 18, 22, 26

7.4: error bounds and iterative refinement: 2, 4, 7

7.5: conjugate gradient: 9, 13.