System Of Linear Equations Three Variables

Solving systems of linear equations with three variables might seem daunting at first, but breaking it down into manageable steps can make the process surprisingly straightforward. This article will guide you through the ins and outs of tackling these systems, equipping you with the knowledge and skills to solve them effectively.

What are Systems of Linear Equations with Three Variables?

A system of linear equations with three variables is a set of three or more linear equations containing three unknowns, typically represented as x, y, and z. The goal is to find the values of these variables that satisfy all equations simultaneously. Geometrically, each equation represents a plane in three-dimensional space, and the solution to the system is the point (or line or plane) where all the planes intersect.

A general form of a linear equation with three variables is:

ax + by + cz = d

where a, b, c, and d are constants, and x, y, and z are the variables. A system of linear equations then looks like this:

a₁x + b₁y + c₁z = d₁ a₂x + b₂y + c₂z = d₂ a₃x + b₃y + c₃z = d₃

Methods for Solving Systems of Linear Equations

Several methods can be employed to solve these systems, with the most common being:

• Substitution Method: Solving one equation for one variable and substituting that expression into the other equations.
• Elimination Method: Adding or subtracting multiples of equations to eliminate one variable at a time.
• Matrix Method: Using matrices and their operations to solve the system. This includes techniques like Gaussian elimination and finding the inverse of a matrix.

Let's explore each of these methods in detail.

1. The Substitution Method

The substitution method involves solving one equation for one variable and then substituting that expression into the other equations. This process reduces the system to one with two variables, which can then be solved using similar techniques. Here's a step-by-step breakdown:

Steps:

1. Solve for one variable: Choose one equation and solve it for one of the variables. Pick the equation and variable that look easiest to isolate. For example, if one equation is x + 2y - z = 5, solving for x might be a good choice: x = 5 - 2y + z.

2. Substitute: Substitute the expression obtained in step 1 into the other two equations. This will result in two equations with two variables.

3. Solve the resulting system: Solve the new system of two equations with two variables. You can use either substitution or elimination.

4. Back-substitute: Once you've found the values of two variables, substitute them back into any of the original equations (or the expression from step 1) to find the value of the third variable.

Example:

Solve the following system of equations using substitution:

x + y + z = 6 (Equation 1) 2x - y + z = 3 (Equation 2) x + 2y - z = 2 (Equation 3)

Solution:

1. Solve for x in Equation 1:

   x = 6 - y - z

2. Substitute this expression for x into Equations 2 and 3:

   Equation 2 becomes: 2(6 - y - z) - y + z = 3 => 12 - 2y - 2z - y + z = 3 => -3y - z = -9 Equation 3 becomes: (6 - y - z) + 2y - z = 2 => 6 + y - 2z = 2 => y - 2z = -4

3. Now we have a system of two equations with two variables:

   -3y - z = -9 (Equation 4) y - 2z = -4 (Equation 5)

   Solve Equation 5 for y:

   y = 2z - 4

4. Substitute this expression for y into Equation 4:

   -3(2z - 4) - z = -9 => -6z + 12 - z = -9 => -7z = -21 => z = 3

5. Back-substitute to find y:

   y = 2(3) - 4 = 6 - 4 = 2

6. Back-substitute to find x:

   x = 6 - y - z = 6 - 2 - 3 = 1

Therefore, the solution to the system is x = 1, y = 2, and z = 3, or (1, 2, 3).

2. The Elimination Method

The elimination method involves adding or subtracting multiples of equations to eliminate one variable at a time. This simplifies the system until you can solve for one variable, and then you can back-substitute to find the others.

Steps:

1. Choose a variable to eliminate: Look for equations where the coefficients of one variable are the same or easily made the same by multiplication.

2. Multiply equations (if necessary): Multiply one or more equations by constants so that the coefficients of the chosen variable are opposites.

3. Add or subtract equations: Add or subtract the equations to eliminate the chosen variable. This will result in a new equation with two variables.

4. Repeat the process: Repeat steps 1-3 with a different pair of equations (or the same pair if necessary) to eliminate the same variable again. You should now have two equations with the same two variables.

5. Solve the resulting system: Solve the new system of two equations with two variables.

6. Back-substitute: Once you've found the values of two variables, substitute them back into any of the original equations to find the value of the third variable.

Example:

Solve the following system of equations using elimination:

2x + y - z = 5 (Equation 1) x - 2y + 3z = -3 (Equation 2) 3x + y + 2z = 4 (Equation 3)

Solution:

1. Eliminate y from Equations 1 and 3: Notice that the coefficient of y is the same in Equations 1 and 3. Subtract Equation 1 from Equation 3:

   (3x + y + 2z) - (2x + y - z) = 4 - 5 => x + 3z = -1 (Equation 4)

2. Eliminate y from Equations 1 and 2: Multiply Equation 1 by 2 to make the coefficient of y equal to 2:

   4x + 2y - 2z = 10 (Equation 1 multiplied by 2) Add this to Equation 2:

   (4x + 2y - 2z) + (x - 2y + 3z) = 10 + (-3) => 5x + z = 7 (Equation 5)

3. Now we have a system of two equations with two variables:

   x + 3z = -1 (Equation 4) 5x + z = 7 (Equation 5)

   Multiply Equation 5 by -3:

   -15x - 3z = -21 Add this to Equation 4:

   (x + 3z) + (-15x - 3z) = -1 + (-21) => -14x = -22 => x = 11/7

4. Back-substitute to find z:

   (11/7) + 3z = -1 => 3z = -1 - (11/7) = -18/7 => z = -6/7

5. Back-substitute to find y (using Equation 1):

   2(11/7) + y - (-6/7) = 5 => 22/7 + y + 6/7 = 5 => y + 28/7 = 5 => y + 4 = 5 => y = 1

Therefore, the solution to the system is x = 11/7, y = 1, and z = -6/7, or (11/7, 1, -6/7).

3. The Matrix Method

The matrix method leverages the power of linear algebra to solve systems of linear equations. This method is particularly useful for larger systems with many variables. Two common techniques within the matrix method are Gaussian elimination and finding the inverse of a matrix.

Gaussian Elimination

Gaussian elimination involves transforming the system of equations into an augmented matrix and then performing row operations to reduce the matrix to row-echelon form or reduced row-echelon form.

Steps:

1. Write the augmented matrix: Represent the system of equations as an augmented matrix.

2. Perform row operations: Use row operations to transform the matrix into row-echelon form or reduced row-echelon form. The allowed row operations are:

   • Swapping two rows.
   • Multiplying a row by a non-zero constant.
   • Adding a multiple of one row to another row.

3. Back-substitution: Once the matrix is in row-echelon form or reduced row-echelon form, use back-substitution to find the values of the variables.

Example:

Solve the following system of equations using Gaussian elimination:

x - y + z = 4 (Equation 1) 2x + y - 3z = -2 (Equation 2) x + y + z = 2 (Equation 3)

Solution:

1. Write the augmented matrix:

   [ 1  -1   1 |  4 ]
   [ 2   1  -3 | -2 ]
   [ 1   1   1 |  2 ]
   

2. Perform row operations to get to row-echelon form:

   • R2 = R2 - 2 * R1:

     [ 1  -1   1 |  4 ]
     [ 0   3  -5 | -10 ]
     [ 1   1   1 |  2 ]
     

   • R3 = R3 - R1:

     [ 1  -1   1 |  4 ]
     [ 0   3  -5 | -10 ]
     [ 0   2   0 | -2 ]
     

   • R3 = R3 / 2:

     [ 1  -1   1 |  4 ]
     [ 0   3  -5 | -10 ]
     [ 0   1   0 | -1 ]
     

   • Swap R2 and R3:

     [ 1  -1   1 |  4 ]
     [ 0   1   0 | -1 ]
     [ 0   3  -5 | -10 ]
     

   • R3 = R3 - 3 * R2:

     [ 1  -1   1 |  4 ]
     [ 0   1   0 | -1 ]
     [ 0   0  -5 | -7 ]
     

   • R3 = R3 / -5:

     [ 1  -1   1 |  4 ]
     [ 0   1   0 | -1 ]
     [ 0   0   1 | 7/5 ]
     

3. Back-substitution:

   • From R3: z = 7/5
   • From R2: y = -1
   • From R1: x - (-1) + (7/5) = 4 => x + 1 + 7/5 = 4 => x = 4 - 1 - 7/5 = 8/5

Therefore, the solution to the system is x = 8/5, y = -1, and z = 7/5, or (8/5, -1, 7/5).

Finding the Inverse of a Matrix

Another matrix method involves finding the inverse of the coefficient matrix. If the system is represented as AX = B, where A is the coefficient matrix, X is the variable matrix, and B is the constant matrix, then the solution is X = A⁻¹B. This method is efficient when you need to solve multiple systems with the same coefficient matrix.

Steps:

1. Write the system in matrix form: Represent the system as AX = B.

2. Find the inverse of matrix A: Calculate the inverse of the coefficient matrix A, denoted as A⁻¹. This involves several steps:

   • Compute the determinant of A. If the determinant is zero, the matrix is singular and has no inverse.
   • Find the matrix of cofactors.
   • Take the transpose of the matrix of cofactors to get the adjugate (or adjoint) of A.
   • Divide the adjugate by the determinant to get A⁻¹.

3. Multiply A⁻¹ by B: Multiply the inverse matrix A⁻¹ by the constant matrix B to find the variable matrix X.

Note: Calculating the inverse of a 3x3 matrix can be computationally intensive, and it's often easier to use software or calculators for this step.

Example:

Solve the following system of equations using the inverse matrix method:

x + 2y + z = 4 (Equation 1) 2x - y + 3z = 9 (Equation 2) x + 3y - z = -2 (Equation 3)

Solution:

1. Write the system in matrix form:

   A = | 1 2 1 | | 2 -1 3 | | 1 3 -1 |

   X = | x | | y | | z |

   B = | 4 | | 9 | | -2 |

2. Find the inverse of matrix A:

   • Determinant of A: det(A) = 1((-1)(-1) - (3)(3)) - 2((2)(-1) - (3)(1)) + 1((2)(3) - (-1)(1)) = 1(1 - 9) - 2(-2 - 3) + 1(6 + 1) = -8 + 10 + 7 = 9

   • Matrix of Cofactors: C = | -8 5 7 | | 5 -2 -1 | | 7 -1 -5 |

   • Adjugate of A (transpose of the matrix of cofactors): adj(A) = | -8 5 7 | | 5 -2 -1 | | 7 -1 -5 |

   • Inverse of A: A⁻¹ = (1/det(A)) * adj(A) A⁻¹ = (1/9) * | -8 5 7 | | 5 -2 -1 | | 7 -1 -5 |

3. Multiply A⁻¹ by B:

   X = A⁻¹B = (1/9) * | -8 5 7 | * | 4 | | 5 -2 -1 | | 9 | | 7 -1 -5 | | -2 |

   X = (1/9) * | (-8)(4) + (5)(9) + (7)(-2) | | (5)(4) + (-2)(9) + (-1)(-2) | | (7)(4) + (-1)(9) + (-5)(-2) |

   X = (1/9) * | -32 + 45 - 14 | | 20 - 18 + 2 | | 28 - 9 + 10 |

   X = (1/9) * | -1 | | 4 | | 29 |

   X = | -1/9 | | 4/9 | | 29/9 |

Therefore, the solution to the system is x = -1/9, y = 4/9, and z = 29/9, or (-1/9, 4/9, 29/9).

Types of Solutions

When solving a system of linear equations with three variables, you can encounter three possible scenarios:

• Unique Solution: The system has one and only one solution. This means the three planes intersect at a single point.
• No Solution: The system has no solution. This can happen if the planes are parallel or if they intersect in such a way that there is no common point of intersection.
• Infinite Solutions: The system has infinitely many solutions. This occurs when the three planes intersect along a line or when all three equations represent the same plane.

Determining the type of solution:

• Unique Solution: The determinant of the coefficient matrix is non-zero.
• No Solution: During the elimination process, you reach a contradiction (e.g., 0 = 1).
• Infinite Solutions: During the elimination process, you end up with an equation where all coefficients and the constant term are zero (e.g., 0 = 0), indicating that the equations are dependent.

Practical Applications

Systems of linear equations with three variables are not just abstract mathematical concepts; they have numerous applications in various fields, including:

• Engineering: Analyzing electrical circuits, structural analysis, and fluid dynamics.
• Physics: Solving problems in mechanics, thermodynamics, and electromagnetism.
• Economics: Modeling supply and demand, analyzing market equilibrium, and optimizing resource allocation.
• Computer Graphics: Creating 3D models, rendering images, and simulating realistic environments.
• Statistics: Performing regression analysis and fitting models to data.
• Chemistry: Balancing chemical equations and determining reaction rates.

Tips for Solving Systems of Linear Equations

• Stay organized: Keep your work neat and organized to avoid errors.
• Check your work: After finding a solution, substitute the values back into the original equations to verify that they satisfy all equations.
• Choose the right method: Consider the specific system of equations and choose the method that seems most efficient. Some systems are easier to solve using substitution, while others are better suited for elimination or matrix methods.
• Practice regularly: The more you practice, the more comfortable you will become with solving these systems.
• Use technology: Utilize calculators, software, or online tools to help with complex calculations and matrix operations.

Conclusion

Solving systems of linear equations with three variables is a fundamental skill with applications across numerous disciplines. By understanding the different methods available – substitution, elimination, and matrix methods – and practicing regularly, you can confidently tackle these problems and apply them to real-world scenarios. Whether you're an engineer, scientist, economist, or student, mastering these techniques will undoubtedly enhance your problem-solving abilities and open doors to new opportunities. Remember to stay organized, check your work, and leverage technology when needed. With dedication and perseverance, you can conquer the world of linear equations and unlock the power of mathematical modeling.