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Consider the following system of equations. $$\left\\{\begin{aligned} x+y &=3 \\\\-2 x-2 y &=-6 \\\\-x-y &=-3 \end{aligned}\right.$$ Use Gauss-Jordan elimination to show that this system Thas infinitely many solutions. Interpret your answer in merms of the graphs of the given equations.

Short Answer

Expert verified
The system of equations given has infinitely many solutions, as they represent the same line graphically. This was concluded by using Gauss-Jordan Elimination, which resulted in row reduced echelon form (rref) of (1,1|3, 1,1|3, 0,0|0) pointing to the fact that all three equations are multiples of each other.

Step by step solution

01

Setup System of Equations and Matrix

Let's write down the system of equations given, \[\begin{align*}x + y &= 3 \-2x - 2y &=-6 \-x - y &=3 \end{align*}\] and rewrite it in matrix notation form or its Augmented Matrix: \( \begin{bmatrix}1 & 1 & | & 3 \-2 & -2 & | & -6 \-1 & -1 & | & 3\end{bmatrix}\)
02

Identify Pivot Rows

All non-zero rows of our defined matrix above pivot rows. They are: row 1, 2 and 3. Thus, no row transformations are necessary at this time.
03

Perform Row Operations

Perform the following row operations to put the augmented matrix into row reduced echelon form (rref). 1) Multiply row 2 by 1/2 and add row 1 to achieve a zero in the upper right of the matrix (new row 2: (1,1,3)), 2) Multiply row 3 by -1 and also add to row 1 (new row 3: (0,0,0)).So the row reduced form of the matrix is:\[\begin{bmatrix}1 & 1 & | & 3 \1 & 1 & | & 3 \0 & 0 & | & 0\end{bmatrix}\]
04

Interpret Results

The last row of the rref matrix (0,0|0) tells us that the equations are dependent, which means that the three equations represent the same line. When we look at our original equations, we can see this to be true (they are multiples of each other). As such, there are infinitely many solutions to this system of equations, as every point on the line they represent is a solution.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

System of Equations
In mathematics, when we refer to a system of equations, we're talking about a collection of two or more equations with a common set of variables. The goal is to find the point or points where all these equations hold true simultaneously. Think of it as searching for a crossroad in a city where all the different streets(intersecting lines) meet.

For example, the provided system involves two equations with two variables, x and y. In geometric terms, each equation represents a line on a graph, and we're looking for where these lines intersect, which would give us the solutions for x and y that satisfy all equations at once.

To solve these, we can use methods like substitution, elimination, or graphical interpretation, but here we focus on what's called Gauss-Jordan elimination. This method systematically transforms the system into a simpler form, ultimately showing us the point of intersection clearly or indicating that there might be other possibilities, like no intersection or an infinite number of intersections.
Row Reduced Echelon Form
The Row Reduced Echelon Form (RREF) represents a matrix that has been transformed using Gauss-Jordan elimination to a state where we can easily read off the solutions to the system of equations it represents. Here are the hallmarks of RREF: each leading entry is 1, leading entries are to the right of those in rows above, rows with all zeros are at the bottom, and each leading entry is the only non-zero number in its column.

When we apply Gauss-Jordan elimination to the given system, our aim is to obtain such a form. By strategically adding, subtracting, and multiplying rows, we can clear out all other numbers in the columns containing the leading 1s. Through these operations, we reveal the essence of the system – whether it has one solution, no solution, or infinitely many solutions.
Dependent Equations
A system has dependent equations when one equation can be derived from another by multiplication or addition—essentially, they are different expressions of the same relationship. When we interpret the RREF of the matrix and see entire rows of zeros, this indicates dependency. The third equation in the system under consideration, after performing Gauss-Jordan elimination, vanishes and becomes a row of zeros.

This indicates that one of our equations was redundant—it didn't provide new information but rather repeated what was already known from the other equations. In terms of graphing these equations, dependent equations represent the same line or plane, so every point on the line (or plane) satisfies all equations. This is why dependent systems can lead to not just one intersection point, but an infinite number.
Infinite Solutions
After transforming a system into RREF and finding dependent equations, we face the possibility that the system has infinite solutions. This happens when the equations describe the same geometric shape—like the same line or the same plane when dealing with more than two dimensions. In our scenario, because the RREF result had a row of zeros and the remaining rows were equivalent, we interpreted this as the system having an infinite number of solutions.

Imagine the original lines overlaying exactly one atop the other—every point where the first line exists is a point where the second line also exists, so there's not just one meeting place but countless. When the system is graphed, we don't see two lines intersecting at a point; instead, we see just one line. Every point on this line is a solution to the system, hence 'infinite solutions'. Understanding this concept is crucial, as it shows us that not all systems are neatly solvable with a single answer, and sometimes, they reveal an unbounded expanse of possibilities.

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Most popular questions from this chapter

Involve the use of matrix multiplication to transform one or more points. This technique, which can be applied to any set of points, is used extensively in computer graphics. Consider a series of points \(\left(x_{0}, y_{0}\right),\left(x_{1}, y_{1}\right),\left(x_{2}, y_{2}\right), \ldots\) such that, for every nonnegative integer \(i,\) the point \(\left(x_{i+1}, y_{i+1}\right)\) is found by applying the matrix \(\left[\begin{array}{cc}1 & -2 \\ 1 & -3\end{array}\right]\) tothe point \(\left(x_{i}, y_{i}\right)\) $$\left[\begin{array}{l}x_{i+1} \\\y_{i+1}\end{array}\right]=\left[\begin{array}{ll}1 & -2 \\\1&-3\end{array}\right]\left[\begin{array}{l}x_{i} \\\y_{i}\end{array}\right]$$ (a) Find \(\left(x_{1}, y_{1}\right)\) if \(\left(x_{0}, y_{0}\right)=(2,-1)\) (b) Find \(\left(x_{2}, y_{2}\right)\) if \(\left(x_{0}, y_{0}\right)=(4,6) .\) (Hint: Find \(\left(x_{1}, y_{1}\right)\) first.) (c) Use the inverse of an appropriate matrix to find \(\left(x_{0}, y_{0}\right)\) if \(\left(x_{3}, y_{3}\right)=(2,3)\)

An electronics firm makes a clock radio in two different models: one (model 380 ) with a battery backup feature and the other (model 360 ) without. It takes 1 hour and 15 minutes to manufacture each unit of the model 380 radio, and only 1 hour to manufacture each unit of the model \(360 .\) At least 500 units of the model 360 radio are to be produced. The manufacturer realizes a profit per radio of \(\$ 15\) for the model 380 and only \(\$ 10\) for the model \(360 .\) If at most 2000 hours are to be allocated to the manufacture of the two models combined, how many of each model should be made to maximize the total profit?

Keith and two of his friends, Sam and Cody, take advantage of a sidewalk sale at a shopping mall. Their purchases are summarized in the following table. $$\begin{array}{lc|c|c|} \hline& {3}{|}\text { Quantity } \\\\\hline\text { Name } & \text { Shirt } & \text { Sweater } & \text { Jacket } \\\\\hline \text { Keith } & 3 & 2 & 1 \\\\\text { Sam } & 1 & 2 & 2 \\\\\text { Cody } & 2 & 1 & 2\\\\\hline\end{array}$$ The sale prices are \(\$ 14.95\) per shirt, \(\$ 18.95\) per sweater, and \(\$ 24.95\) per jacket. In their state, there is no sales tax on purchases of clothing. Use matrix multiplication to determine the total expenditure of each of the three shoppers.

This set of exercises will draw on the ideas presented in this section and your general math background. Find the inverse of $$\left[\begin{array}{lll}a & a & a \\\0 & 1 & 0 \\\0 & 0 & 1\end{array}\right]$$ where \(a\) is nonzero. Evaluate this inverse for the case in which \(a=1\)

In this set of exercises, you will use the method of solving linear systems using matrices to study real-world problems. A gardener, is mixing organic fertilizers consisting of bone meal, cottonseed meal, and poultry manure. The percentages of nitrogen (N), phosphorus (P), and potassium (K) in each fertilizer are given in the table below. $$\begin{array}{lccc}\hline & \begin{array}{c}\text { Nitrogen } \\\\(\%)\end{array} & \begin{array}{c}\text { Phosphorus } \\\\(\%)\end{array} & \begin{array}{c}\text { Potassium } \\\\(\%)\end{array} \\\\\hline \text { Bone meal } & 4 & 12 & 0 \\\\\text { Cottonseed meal } & 6 & 2 & 1 \\\\\text { Poultry manure } & 4 & 4 & 2\end{array}If Mr. Greene wants to produce a 10 -pound mix containing \(5 \%\) nitrogen content and \(6 \%\) phosphorus content, how many pounds of each fertilizer should he use?

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