Problem 9.4 Given the system of equations -3x2 + 7x3 = 4 x1 + 2x2 - x3 = 0 5x1 -2 x2 = 3 (a) Compute the determinant. (b) Use Cramer’s rule to solve for the x’s. (c) Use Gauss elimination with partial pivoting to solve for the x’s. As part of the computation, calculate the determinant in order to verify the value computed in (a) (d) Substitute your results back into the original equations to check your solution.

- 3 x₂ + 7 x₃ = 3

x₁ + 2 x₂ - x₃ = 0

5 x₁ - 2 x₂ = 3

(a) I suppose this is asking for the determinant of the coefficient matrix.

[tex]\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}[/tex]

Using a cofactor expansion along the first row, this reduces to

[tex]0\begin{vmatrix}2&-1\\-2&0\end{vmatrix}-(-3)\begin{vmatrix}1&-1\\5&0\end{vmatrix}+7\begin{vmatrix}1&2\\5&-2\end{vmatrix}= 3(0 - (-5))+7(-2-10)=\boxed{-69}[/tex]

(b) Using Cramer's rule, we have

[tex]x_1=\dfrac{\begin{vmatrix}\mathbf 3&-3&7\\\mathbf 0&2&-1\\\mathbf 3&-2&0\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]

[tex]x_2=\dfrac{\begin{vmatrix}0&\mathbf 3&7\\1&\mathbf 0&-1\\5&\mathbf 3&0\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]

[tex]x_3=\dfrac{\begin{vmatrix}0&-3&\mathbf 3\\1&2&\mathbf 0\\5&-2&\mathbf 3\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]

That is, solving for n-th variable consists of dividing [the determinant of the coefficient matrix with its n-th column replaced with the right side of the system, the numbers in boldface,] by [the determinant of the coefficient matrix].

Compute each determinant:

[tex]\begin{vmatrix}\mathbf 3&-3&7\\\mathbf 0&2&-1\\\mathbf 3&-2&0\end{vmatrix}=3\begin{vmatrix}2&-1\\-2&0\end{vmatrix}+3\begin{vmatrix}-3&7\\2&-1\end{vmatrix}=3(0-2)+3(3-14)=-39[/tex]

(expanding along the first column)

[tex]\begin{vmatrix}0&\mathbf 3&7\\1&\mathbf 0&-1\\5&\mathbf 3&0\end{vmatrix}=-1\begin{vmatrix}3&7\\3&0\end{vmatrix}+5\begin{vmatrix}3&7\\0&-1\end{vmatrix}=-1(0-21)+5(-3-0)=6[/tex]

(again, along the first column)

[tex]\begin{vmatrix}0&-3&\mathbf 3\\1&2&\mathbf 0\\5&-2&\mathbf 3\end{vmatrix}=-1\begin{vmatrix}-3&3\\-2&3\end{vmatrix}+5\begin{vmatrix}-3&3\\2&0\end{vmatrix}=-1(-9-(-6))+5(0-6)=-27[/tex]

(first column)

So, we get the solution

[tex]x_1=\dfrac{-39}{-69}=\boxed{\dfrac{13}{23}},x_2=\dfrac6{-69}=\boxed{-\dfrac2{23}},x_3=\dfrac{-27}{-69}=\boxed{\dfrac9{23}}[/tex]

(c) Using elimination:

- 3 x₂ + 7 x₃ = 3

x₁ + 2 x₂ - x₃ = 0

5 x₁ - 2 x₂ = 3

Swap the first two equations:

x₁ + 2 x₂ - x₃ = 0

- 3 x₂ + 7 x₃ = 3

5 x₁ - 2 x₂ = 3

Add -5(equation 1) to equation 3:

x₁ + 2 x₂ - x₃ = 0

- 3 x₂ + 7 x₃ = 3

- 12 x₂ + 5 x₃ = 3

Add -4(equation 2) to equation 3:

x₁ + 2 x₂ - x₃ = 0

- 3 x₂ + 7 x₃ = 3

- 23 x₃ = -9

Multiply through equation 3 by -1/23:

x₁ + 2 x₂ - x₃ = 0

- 3 x₂ + 7 x₃ = 3

x₃ = 9/23

Add -7(equation 3) to equation 2:

x₁ + 2 x₂ - x₃ = 0

- 3 x₂ = 6/23

x₃ = 9/23

Multiply through equation 2 by -1/3:

x₁ + 2 x₂ - x₃ = 0

x₂ = -2/23

x₃ = 9/23

Add -2(equation 2) and equation 3 to equation 1:

x₁ = 13/23

x₂ = -2/23

x₃ = 9/23

4Flaming Hoop Jump of Awesome Daredevil Danny attempts the feat of jumping his motorcycle through the Flaming Hoop Jump of Awesome. In order for Daredevil Danny to pass through the hoop, he will need a safe path to travel. Let’s explore the parabolic trajectory that he will need for a safe journey. Directions Complete each of the following tasks, reading the directions carefully as you go. Be sure to show all work where indicated, including inserting images of graphs. Be sure that all graphs or screenshots include appropriate information such as titles, labeled axes, etc. If your word processing program has an equation editor, you can insert your equations here. Otherwise, print this activity sheet and write your answers by hand. In addition to the answers you determine, you will be graded based on the work you show, or your solution process. So, be sure to show all your work and answer each question as you complete the task. Type all your work into this document so you can submit it to your teacher for a grade. You will be given partial credit based on the work you show and the completeness and accuracy of your explanations. Your teacher will give you further directions about how to submit your work. You may be asked to upload the document, e-mail it to your teacher, or print it and hand in a hard copy. Now, let’s get started! Step 1: Analyze the graph. Daredevil Danny takes a practice jump as shown on the graph below. Complete the steps to determine the equation for the parabolic trajectory of his practice jump. a) What are the key aspects of any parabola? What do the key aspects tell you about the graph? (5 points) b) What are the key aspects of Daredevil Danny’s practice jump shown in the graph provided? Describe each value and how it is determined. (10 points) Step 2: Solve for coefficient a. a) What equation is used to find the vertex form of a parabola with the vertex (h, k)? (5 points) b) Solve for the coefficient a in the vertex form for the parabola in Daredevil Danny’s practice jump using key aspects from step 1. As you solve for the coefficient, explain each step of the process in detail. (15 points) c) What is the vertex form of the parabola from Daredevil Danny’s practice jump? (5 points) Step 3: Change from vertex form to standard form. To get Daredevil Danny through the Flaming Hoop Jump of Awesome, you will need to know the values of a, b, and c, which are the coefficients of the quadratic equation in standard form. a) What is the equation for a parabola in standard form? (5 points) b) Describe how to convert from vertex form to standard form. (10 points c) Convert the vertex form of the equation you wrote above to standard form. (10 points) d) What are the values for coefficients a, b, and c in the quadratic equation for Daredevil Danny’s practice jump? (5 points) Step 4: Calculate the equation for the flaming hoop jump. Using the same method for the practice jump, write the standard form of the equation that will get Daredevil Danny safely through the Flaming Hoop Jump of Awesome! a) Write the vertex form by substituting in values for x, y, h, and k, and then solve for a. Show your work. (10 points) b) Determine the values of a, b, and c. Show your work. (10 points) c) Using your values of a, b, and c, did Daredevil Danny successfully pass through the Flaming Hoop Jump of Awesome? If not, describe what happened. Then, go back to critique your work and revise it. Write about your revision. (5 points) d) Once you have determined the correct values of a, b, and c, verify algebraically that Daredevil Danny will successfully pass through the Flaming Hoop Jump of Awesome. (5 points)

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