Pearson Algebra 2 Common Core, 2011
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Pearson Algebra 2 Common Core, 2011 View details
5. Systems With Three Variables
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Exercise 11 Page 171

Can you manipulate the coefficients of any variable terms such that they could be eliminated?

(2,1,-5)

Practice makes perfect

The given system consists of equations of planes. Notice that the coefficient of z in the first equation is the additive inverse of the coefficient of z in the second equation; they will add to be 0. Let's use the Elimination Method to find a solution to this system. -2x+y - z=2 & (I) - x-3y + z=-10 & (II) 3x+6z=-24 & (III) We can start by adding the second equation to the first equation to eliminate the z-terms.

-2x+y-z=2 & (I) - x-3y+z=-10 & (II) 3x+6z=-24 & (III)
-2x+y-z+( - x-3y+z)=2+( -10) - x-3y+z=-10 3x+6z=-24
-2x+y-z-x-3y+z=2-10 - x-3y+z=-10 3x+6z=-24
-3x-2y=-8 - x-3y+z=-10 3x+6z=-24

Having eliminated the z-variable from the first equation, we can continue by creating additive inverse coefficients for z in the second and third equations. Then, we can add or subtract these equations to eliminate z from the third equation.

-3x-2y=-8 - x-3y+z=-10 3x+6z=-24
-3x-2y=-8 6x+18y-6z=60 3x+6z=-24
-3x-2y=-8 6x+18y-6z+( 3x+6z)=60+( -24) 3x+6z=-24
-3x-2y=-8 6x+18y-6z+3x+6z=60-24 3x+6z=-24
-3x-2y=-8 9x+18y=36 3x+6z=-24
-3x-2y=-8 x+2y=4 3x+6z=-24

Next, we will use our two equations that are only in terms of x and y to solve for the value of one of the variables. We will once again apply the Elimination Method, but this time it will be similar to when using it in a system with only two variables.

-3x-2y=-8 x+2y=4 3x+6z=-24
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(II): Solve by elimination
-3x-2y=-8 x+2y+( -3x-2y)=4+( -8) 3x+6z=-24
-3x-2y=-8 x+2y-3x-2y=4-8 3x+6z=-24
-3x-2y=-8 -2x=-4 3x+6z=-24
-3x-2y=-8 x=2 3x+6z=-24

Now that we know that x=2, we can substitute it into the first equation to find the value of y and into the third equation to find z.

-3x-2y=-8 x=2 3x+6z=-24

(I), (III): x= 2

-3( 2)-2y=-8 x=2 3( 2)+6z=-24

(I), (III): Multiply

-6-2y=-8 x=2 6+6z=-24
-2y=-2 x=2 6+6z=-24
y=1 x=2 6+6z=-24
y=1 x=2 6z=-30
y=1 x=2 z=-5

The solution to the system is ( 2, 1, -5). This is the singular point at which all three planes intersect. Now, we can check our solution by substituting the values into the system.

-2x+y-z=2 & (I) - x-3y+z=-10 & (II) 3x+6z=-24 & (III)

(I), (II), (III): Substitute values

-2( 2)+ 1-( -5)? =2 -( 2)-3( 1)+( -5)? =-10 3( 2)+6( -5)? =-24

(I), (II), (III): Multiply

-4+1-(-5)? =2 -(2)-3+(-5)? =-10 6-30? =-24

(I), (II), (III): Add and subtract terms

2=2 ✓ -10=-10 ✓ -24=-24 ✓

Since the substitution of our answers into the given equations resulted in three identities, we know that our solution is correct.