Question on reducing 3 equations with 3 unknowns

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Discussion Overview

The discussion revolves around a system of three equations with three unknowns, specifically focusing on the possibility of reducing the equations to fewer variables. Participants explore methods for transforming the equations to either two variables or one variable, considering both linear and quadratic forms.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires whether it is possible to reduce the system of equations to two or one variable.
  • Another participant suggests replacing the squared terms with new variables (AA, BB, CC) to facilitate solving the equations using matrix methods.
  • A different participant references a document that discusses reducing three quadratic equations to either two second-order equations or one quartic equation, indicating an interest in understanding this reduction process.
  • One participant proposes treating the squares of the unknowns as new variables (X, Y, Z) and explains that the equations can be simplified by eliminating one variable at a time, providing a method to derive equations in fewer variables.
  • Further elaboration includes specific algebraic manipulations to eliminate variables, such as solving for one variable and substituting it into the other equations to derive a new system.

Areas of Agreement / Disagreement

Participants express various methods for reducing the equations, but there is no consensus on a single approach or solution. Multiple competing views on how to achieve the reduction remain present throughout the discussion.

Contextual Notes

Some participants' methods depend on specific conditions, such as the non-zero nature of certain coefficients, which may affect the applicability of their proposed solutions. Additionally, the discussion does not resolve the mathematical steps involved in the reduction process.

majon
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Say I have a system of equations of the following form:

a_1 A^2 + b_1 B^2 + c_1 C^2 + d_1 = f_1
a_2 A^2 + b_2 B^2 + c_2 C^2 + d_2 = f_2
a_3 A^2 + b_3 B^2 + c_3 C^2 + d_3 = f_3

Where a,b,c,d,f are coefficients, and A,B and C are unknown variables.

1. Can we write them in terms of two variables only?
2. Can we write them in terms of one variable only?
 
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If you replace the A^2 with AA and similarly with B and C then you have 3 linear eqns in AA, BB, and CC that can be easily solved using matrix methods.
 
I was asking because I was reading a document where the author deals with 3 equations and 3 unknown parameters, each equation has quadratic dependence on each of the 3 parameters. Then the author says: with these three equations and three parameters we can reduce the equations to two second order equations with respect to two unknowns (call this case 1), or to one quartic equation with respect to one unknown (call this case 2). So I wanted to know how to do this. I'll do the exercise by eliminating one of the variable and see what I get.
 
Ad jedishrfu said, in the given equation, since only the squares of the unknown values occur, we can treat them as, say X=A^2, Y= B^2, and Z= C^2. Then the equations become a_1X+ b_1Y+ c_1Z= f_1, a_2X+ b_2Y+ c_2Z= f_2, and a_3X+ b_3Y+ c_3Z= f_3. (There is no need for the separate 'd_1' and 'f_1'- we can always subtract d_1 from both sides of the equation.)

There are many ways to solve a "system of equations" but, yes, they all basically involve reducing from three equations in three unknowns to two equations in two unknowns and then to one equation in one unknown. For example, if a_2 is non-zero, I can solve the first equation for X: a_1X= f_1- b_1Y- c_1Z so X= (f_1-b_1X- c_1Z)/a_1. Now replace "X" in the other two equations by that to get two equations in Y and Z only.

Or:
1) multiply the first equation by a_2 to get
a_1a_2X+ b_1a_2Y+ c_1a_2Z= f_1a_2 and
2) multiply the second equation by a_1 to get
a)a_1a_2X+ a_1b_2Y+ a_2c_3Z= a_2f_3. Now
3) subtract those two equation. Since the X term in each equation has the same coefficient, a_1a_2, they cancel leaving a single equation in Y and Z.

Do the same with, say, the first and third equation, to get a second equation in Y and Z.
 

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