Big Ideas Math: Modeling Real Life, Grade 8
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7. Writing Equations in Point-Slope Form
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Exercise 30 Page 184

Practice makes perfect

We are told that the Leaning Tower of Pisa in Italy was built between 1173 and 1350. Consider the following sketch of the tower.

Let's consider y as the height and x as the distance from the center. We want to write an equation that represents the yellow line. To do so, remember that equations written in point-slope form follow a specific format. y- y_1= m(x- x_1) In this form, m is the slope of the line and ( x_1, y_1) is a point on the line. From the given draw can see that the line passes through two marked points. Let's start by substituting the points given on the graph, (7.75,0) and (10.75,42), into the slope formula to calculate m.
m = y_2-y_1/x_2-x_1
m=42- 0/10.75- 7.75
â–Ľ
Simplify right-hand side
m=42/3
m= 14
Now that we know the slope of the line is 14, we can write the equation of the line in point-slope form. We can use either of the given points as (x_1,y_1) in our equation. Let's use ( 0, 7.75).
y-y_1=m(x-x_1)
y- 0= 14(x- 7.75)
y=14(x-7.75)
y=14x-(14)(7.75)
y=14x-108.5
Please note that any point on the line can be used to form a point-slope equation. Therefore, our equation is only one of infinitely many possible equations!
We want to know how far is the top of the tower from the center if the tower is 56 meters tall. To do so, let's use the equation obtained in Part A. y=14x-108.5 Since y represents the height and x is the distance from the center, we need to start by substituting y= 56 into the above equation and solve it for x.
y=14x-108.5
â–Ľ
Solve for x
56=14x-108.5
56+108.5=14x-108.5+108.5
164.5=14x
164.5/14=14x/14
164.5/14=x
11.75=x
x=11.75
The tower is 11.75 meters from the center.