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In-Class Assignment 1

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Linear Algebra

Instructor: Le Hai Yen


In-Class Assignment 1 October 25, 2022

Students:

Recall: Common method to solve a linear system using Gaussian/Gauss-Jordan elimination:

1) Construct a matrix (called the augmented matrix) corresponding to the system

2) Using elementary row operations on matrices to reduce the augmented matrix to an upper triangular
matrix.
3) Using back-substitution to solve the final system.

Question 1. Which of the following are linear equations in x1 , x2 and x3 ?


√ √
1. x1 + 5x2 − x3 = 1 3. πx1 − 2x2 + 31 x3 = 71/3
√ √
2. x−2
1 + x2 + 8x3 = 5 4. x1 − 3x2 + 4−√
5
32
x3 = 2

Question 2. The following matrices are the augmented matrices of systems of linear equations in variables
x1 , x2 , . . .. Write down the corresponding systems explicitly and solve them.

1. 2. 3. 4.
 
    1 6 0 0 4 −2  
1 0 0 5 1 0 0 4 −1 0
 0 1 0 3 1 1 2 0 3 0 7
0 1 0 −2 0 1 0 2 6  0 0 0 1 5 2 0 0 1 0 0 1
0 0 1 4 0 0 1 3 2 0 0 0 0 0 0 0 0 0 0 1 2
Linear Algebra
Instructor: Le Hai Yen
In-Class Assignment 1 October 25, 2022

Question 3. Solve the following linear systems using Gaussian or Gauss-Jordan (back-substitution) elimination.
Write clearly all elementary row operations you used.

1. 2. 3.

x + y + 2z = 0 2x + 2y + 2z = 0 x + 2y − t + w = 1
2x + 4y − 3z = 1 −2x + 5y + 2z = 1 3y + z − w = 2
3x + 6y − 5z = 0. 8x + y + 4z = −1. z + 7t = 1.
Linear Algebra
Instructor: Le Hai Yen
In-Class Assignment 1 October 25, 2022

Question 4. For which values of a will the following system have no solutions? Exactly one solution? Infinitely
many solutions?

x + 2y − 3z = 4
3x − y + 5z = 2
4x + y + (a2 − 14)z = a + 2.

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