Big Ideas Math: Modeling Real Life, Grade 8
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1. Solving Systems of Linear Equations by Graphing
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Exercise 27 Page 204

Recall the formula for the perimeter of a rectangle. How can we use it to create a system of linear equations?

14 feet

Practice makes perfect
We want to find the width of a rectangle. We can write a system of linear equations and solve it for the width of the rectangle. First, let's recall the formula for the perimeter of a rectangle. P=2l+2w In this formula, P is the perimeter of the rectangle, l is the length, and w is the width. We know from the exercise that the perimeter of the rectangle is 72 feet. We also know that the length of our rectangle is 8 feet more that its width. l=w+ 8 Now let's create a system of linear equations. We will rewrite the formula for the perimeter of the rectangle to isolate l on one side of the equation. Let's do it!
P=2l+2w
72=2l+2w
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Solve for l
72-2w=2l
36-w=l
l=36-w
Now we are ready to write the system of equations. l=36-w & (I) l=w+8 & (II) Next, we will graph each equation and look for a point of intersection, which will be the solution to the system of equations.
The graphs seem to intersect at the point (14,22). Finally, let's check if this point is a solution to the system of equations we createdby substituting 14 for w and 22 for l in each equation and checking to see if they produce true statements.
l=36-w & (I) l=w+8 & (II)
22? =36-( 14) 22? = 14+8
22=22 âś“ 22? =14+8
22=22 âś“ 19=19 âś“
We found that the solution to the system is (14,22). This means that the width of the rectangle is 14 feet!