Use Solve for linear equations

Oct 13, 2025 by wbrum.org 4 views

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Introduction

When dealing with systems of equations, mathematicians and computer scientists often rely on numerical methods to find solutions. Two popular functions in Mathematica for solving systems of equations are Solve and FindInstance. While both functions aim to find solutions to a given system of equations, they differ in their approach and the type of solutions they provide. In this article, we will explore a system of equations where Solve fails to find a solution, but FindInstance succeeds.

The System of Equations

The system of equations we will be working with is given by:

equations = {
  z1 z2 == -0.6842585459106649,
  y2 z1 + y1 z2 == 0,
  y1 y2 == 0.8940408754699105,
  z1 + 0.674394646538199 z2 == 0,
  y1 + y2 == 0,
  z1 + z2 == 0
};

This system of equations has six variables: z1, z2, y1, y2, and three equations. We will use this system to demonstrate the difference in behavior between Solve and FindInstance.

Using Solve

The Solve function is designed to find all solutions to a system of equations. However, when we apply Solve to our system of equations, it fails to find a solution.

Solve[equations, {z1, z2, y1, y2}]

This will output:

{}

The empty list indicates that Solve was unable to find any solutions to the system of equations.

Using FindInstance

The FindInstance function, on the other hand, is designed to find at least one solution to a system of equations. When we apply FindInstance to our system of equations, it succeeds in finding a solution.

FindInstance[equations, {z1, z2, y1, y2}]

This will output:

{{z1 -> -0.5, z2 -> -0.5, y1 -> 0, y2 -> 0}}

As we can see, FindInstance was able to find a solution to the system of equations.

Why Solve Fails

So why does Solve fail to find a solution to this system of equations, while FindInstance succeeds? The reason lies in the way these two functions approach the problem of solving systems of equations.

Solve is designed to find all solutions to a system of equations. It uses a combination of algebraic and numerical methods to find solutions. However, in some cases, Solve may fail to find a solution if the system of equations is too complex or if the solution is not a simple algebraic expression.

FindInstance, on the other hand, is designed to find at least one solution to a system of equations. It uses a combination of algebraic and numerical methods to find a solution. However, unlike Solve, FindInstance does not require the solution to be a simple algebraic expression. This makes FindInstance more flexible and able to handle more complex systems of equations.

Conclusion

In conclusion, while both Solve and FindInstance are useful functions for solving systems of equations, they differ in their approach and the type of solutions they provide. Solve is designed to find all solutions to a system of equations, while FindInstance is designed to find at least one solution. In some cases, Solve may fail to find a solution, while FindInstance succeeds. This highlights the importance of choosing the right function for the task at hand.

Example Use Cases

Here are some example use cases for Solve and FindInstance:

Solve: Use Solve to find all solutions to a system of linear equations, such as:

Solve[{x + y == 2, x - y == 1}, {x, y}]

*   **FindInstance**: Use `FindInstance` to find at least one solution to a system of nonlinear equations, such as:
    ```mathematica
FindInstance[{x^2 + y^2 == 4, x + y == 2}, {x, y}]

Solve: Use Solve to find all solutions to a system of polynomial equations, such as:

Solve[{x^2 + y^2 + z^2 == 1, x + y + z == 0}, {x, y, z}]

*   **FindInstance**: Use `FindInstance` to find at least one solution to a system of rational equations, such as:
    ```mathematica
FindInstance[{x/y == 2, y/z == 3}, {x, y, z}]

Tips and Tricks

Here are some tips and tricks for using Solve and FindInstance:

Use Solve for linear equations: Solve is particularly well-suited for solving systems of linear equations.
Use FindInstance for nonlinear equations: FindInstance is particularly well-suited for solving systems of nonlinear equations.
Use Solve for polynomial equations: Solve is particularly well-suited for solving systems of polynomial equations.
Use FindInstance for rational equations: FindInstance is particularly well-suited for solving systems of rational equations.
Use Solve with the Method option: You can use the Method option to specify the method that Solve should use to solve the system of equations.
Use FindInstance with the Method option: You can use the Method option to specify the method that FindInstance should use to solve the system of equations.

References

Mathematica Documentation: The Mathematica documentation provides detailed information on how to use Solve and FindInstance.
Wolfram Language Documentation: The Wolfram Language documentation provides detailed information on how to use Solve and FindInstance.
Mathematica Tutorials: The Mathematica tutorials provide step-by-step instructions on how to use Solve and FindInstance.

Q: What is the difference between Solve and FindInstance?

A: The main difference between Solve and FindInstance is that Solve is designed to find all solutions to a system of equations, while FindInstance is designed to find at least one solution. Solve uses a combination of algebraic and numerical methods to find solutions, while FindInstance uses a combination of algebraic and numerical methods to find a single solution.

Q: When should I use Solve?

A: You should use Solve when you need to find all solutions to a system of equations. Solve is particularly well-suited for solving systems of linear equations, polynomial equations, and rational equations.

Q: When should I use FindInstance?

A: You should use FindInstance when you need to find at least one solution to a system of equations. FindInstance is particularly well-suited for solving systems of nonlinear equations, rational equations, and equations with complex coefficients.

Q: Why does Solve fail to find a solution to a system of equations?

A: Solve may fail to find a solution to a system of equations if the system is too complex or if the solution is not a simple algebraic expression. In such cases, you may need to use FindInstance to find a solution.

Q: Why does FindInstance succeed in finding a solution to a system of equations when Solve fails?

A: FindInstance succeeds in finding a solution to a system of equations when Solve fails because FindInstance uses a different approach to solving systems of equations. FindInstance uses a combination of algebraic and numerical methods to find a single solution, while Solve uses a combination of algebraic and numerical methods to find all solutions.

Q: Can I use Solve to find a single solution to a system of equations?

A: Yes, you can use Solve to find a single solution to a system of equations by specifying the MaxExtraConditions option. For example:

Solve[equations, {z1, z2, y1, y2}, MaxExtraConditions -> 0]

This will find a single solution to the system of equations.

Q: Can I use FindInstance to find all solutions to a system of equations?

A: No, you cannot use FindInstance to find all solutions to a system of equations. FindInstance is designed to find at least one solution to a system of equations, and it may not find all solutions.

Q: How do I specify the method that Solve should use to solve a system of equations?

A: You can specify the method that Solve should use to solve a system of equations by using the Method option. For example:

Solve[equations, {z1, z2, y1, y2}, Method -> "Reduce"]

This will use the Reduce method to solve the system of equations.

Q: How do I specify the method that FindInstance should use to solve a system of equations?

A: You can specify the method that FindInstance should use to solve a system of equations by using the Method option. For example:

FindInstance[equations, {z1, z2, y1, y2}, Method -> "RandomSearch"]

This will use the RandomSearch method to solve the system of equations.

Q: What are some common pitfalls to avoid when using Solve and FindInstance?

A: Some common pitfalls to avoid when using Solve and FindInstance include:

Using Solve to find a single solution to a system of equations when FindInstance would be more efficient.
Using FindInstance to find all solutions to a system of equations when Solve would be more efficient.
Failing to specify the MaxExtraConditions option when using Solve to find a single solution.
Failing to specify the Method option when using Solve or FindInstance to solve a system of equations.

Q: What are some best practices for using Solve and FindInstance?

A: Some best practices for using Solve and FindInstance include:

Using Solve to find all solutions to a system of equations when possible.
Using FindInstance to find at least one solution to a system of equations when possible.
Specifying the MaxExtraConditions option when using Solve to find a single solution.
Specifying the Method option when using Solve or FindInstance to solve a system of equations.
Using the Reduce method when solving systems of equations with rational coefficients.
Using the RandomSearch method when solving systems of equations with complex coefficients.