Big Ideas Math: Modeling Real Life, Grade 8
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2. Solving Systems of Linear Equations by Substitution
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Exercise 18 Page 210

Think of two equations that are satisfied by the given point.

Example Solution: 2x+3y=20 4x+2y=16

Practice makes perfect

We want to write a system of linear equation that has the ordered pair (1,6) as its solution. To do so, we need to think of two equations that are satisfied by the given point. ? x+? y=? & (I) ? x+? y=? & (II) Let's choose coefficients of x and y in both equations. Since we want system to have one solution we should not choose the same coefficients for both equations. 2x+3y=? 4x+2y=? Now, to evaluate right-hand side for each equation we should substitute ( 1, 6) for x and y into the left-hand side and simplify. By doing this, we make sure that each equation is satisfied with the given point.

Equation 1 Equation 2
Substitute 2( 1)+3( 6) 4( 1)+2( 6)
Multiply 2+18 4+12
Add 20 16

Finally we can complete our system. Keep in mind that this is just one possible answer for this situation. 2x+3y= 20 4x+2y= 16

Checking Our Answer

Checking Our Solution
Let's check if our example system has a solution of (1,6). To do this, let's solve our system by substitution. We will start by solving Equation 2 for y.

2x+3y=20 4x+2y=16
2x+3y=20 2y=16-4x
2x+3y=20 y=16-4x/2
2x+3y=20 y=16/2-4x/2
2x+3y=20 y=8-2x

Now we will substitute the right-hand side of Equation 2 for y into Equation 1.

2x+3y=20 y=8-2x
2x+3( 8-2x)=20 y=8-2x
2x+3*8-3*2x=20 y=8-2x
2x+24-6x=20 y=8-2x
-4x+24=20 y=8-2x
-4x=-4 y=8-2x
x=1 y=8-2x

Finally, we will substitute 1 for x into Equation 2.

x=1 y=8-2x
x=1 y=8-2( 1)
x=1 y=8-2
x=1 y=6

The solution to our system is (1,6), so we are correct.