Solve system of three equations using elimination

In summary, using elimination to solve a system of three equations in three unknowns involves eliminating one unknown at a time by combining two equations at a time. Once two equations are eliminated, the remaining two equations can be used to eliminate the second unknown, leaving one equation in one unknown. The solution must satisfy all three equations in order to be correct.
  • #1
cscott
782
1
How can I use elimination to solve a system of three equations that are 3-dimensional using elimination? When I do it the point I find doesn't work for all three equations.
 
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  • #2
You did exactly the right thing when you checked your solution with the orginal equations. As to why you get the wrong answer, it would be impossible to say unless you showed us what you did. My guess is that you made an algebraic error somewhere in your work.
 
  • #3
You use elimination for three equations exactly the way you would for two. Use two of the equations together to eliminate one unknown. Then use one of those equations together with the third equation to eliminate that same unknown. Now you have two equations in two unknowns. Use those to eliminate a second unknown leaving you with one equation in one unknown. Solve that, then substitute back into the equations to find the other two. If that solution doesn't work for all three equations, then you are doing something wrong!

Example: x+ y+ z= 0, x- 2y- z= 0, x+ y- z= 6.

If we add both sides of x+ y+ z= 0 and x- 2y-z= 0, we eliminate z and get 2x- y= 0. If we add both sides of x+ y+ z= 0 to x+ y- z= 6, we again eliminate z and get 2x+ 2y= 6 which is the same as x+ y= 3. We now have the two equations 2x-y= 0 and x+ y= 3. If we add both sides of 2x- y= 0 and x+ y= 3 we eliminate y and have 3x= 3. Divide both sides of that last equation and we get x= 1. Put that into either 2x- y= 0, so that 2- y= 0, or x+ y= 3, so that 1+ y= 3, and we get y= 2. Put x= 1, y= 2 into any of the original equations and we get 1+ 2+ z= 0 or 1- 4- z= 0 or 1+ 2- z= 6, all of which lead to z= -3. The point (1, 2, -3) satisfies all three equations.
 
  • #4
Thanks for the example. I can't seem to get the correct y and x coordinates with my equations:

(1) 3x + 6y + 9z + 18 = 0, (2) x + 4y + 8z + 2 = 0, (3) x + 3y + 4z + 6 = 0

Subtract (2) and (3) to get y + 4z - 4 = 0, and subtract (1) and (3)*3 to get -3y - 3z = 0 or y = -z

(-z) + 4z -4 = 0
z = 3/4

But y and x depend on which equations a plug my known values into...
 

FAQ: Solve system of three equations using elimination

What is the purpose of solving a system of three equations using elimination?

The purpose of solving a system of three equations using elimination is to find the values of the variables that satisfy all three equations simultaneously. This allows us to find the intersection point, or points, of three lines, which can have various real-world applications in fields such as engineering, economics, and physics.

What is the process of solving a system of three equations using elimination?

The process involves eliminating one variable at a time by multiplying one or both equations by a constant, and then adding or subtracting the equations to eliminate that variable. This is repeated until only two equations with two variables remain, which can then be solved using substitution or another method.

What are some tips for solving a system of three equations using elimination?

Some tips include choosing which variable to eliminate first based on coefficients (choose the variable with the smallest coefficient), making sure to multiply both equations by the same constant when eliminating, and checking the solutions by plugging them back into the original equations.

Can a system of three equations always be solved using elimination?

No, a system of three equations cannot always be solved using elimination. This method only works when there are three equations with three variables, and the equations are linear (no exponents). If the system is inconsistent (no solutions) or dependent (infinite solutions), elimination will not work.

What are some real-world applications of solving a system of three equations using elimination?

Solving a system of three equations using elimination can be useful in fields such as engineering (finding the intersection point of three electrical currents), economics (determining the optimal production levels for three different products), and physics (finding the point where three forces intersect). It can also be used in everyday life, such as finding the intersection point of three streets on a map.

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