Algebra 1 Common Core Student Edition, Grade 8-9

How do you draw an
accurate trend line?
An accurate trend line
should fit the data
closely. There should be
about the same number
of points above the line
as below it.

mini
How can you check
the reasonableness
of your answer?
Since x = 7 is visible
on the graph, find its
corresponding y-value.
Whenx = 7 ,y = 32.7.
So the estimate is
reasonable.

Writing an Equation of a Trend Line

Biology Make a scatter plot of the data at the right. What is the

approximate weight of a 7-month-old panda?

Weight of a Panda

Step 1 Make a scatter plot a n d draw a tre n d line. Estim ate the

coordinates of two points on th e line.

Weight of a Panda

1W 1W H

(m onths) | a

(^1) 2.5
2 7.6
3 12.5
4 17.1
(^6) 24.3
8 37.9
(^10) 49.2
12 54.9
Age (months)
Two points on the tre n d line are (4,17.1) a n d (8, 37.9).
Step 2 W rite a n eq u a tio n of th e tre n d line.
m = ;. T2 ~y\ _ 37.9 - 17.18-4 20.8 4 5.2 Find the slope of the trend line.
Use point-slope form.
Substitute 5.2 for m and (4,17.1) for ( x ^ y ,).
Distributive Property
Add 17.1 to each side.
y - y l = m { x - xq)
y - 17.1 = 5. 2 0 - 4 )
y - 17.1 = 5. 2 x - 20.8
y = 5. 2 x - 3.7
Step 3 Estim ate th e weight of a 7 -m onth-old panda.
y = 5.2(7) - 3.7 Substitute 7 forx.
y = 32.7 Simplify.
The weight of a 7 -m onth-old p a n d a is ab o u t 32.7 lb.
Got It? 2. a. Make a scatter plot of th e d ata below. Draw a tren d line a n d w rite its
equation. W hat is th e approxim ate body length of a 7 -m onth-old p anda?
Body Length of a Panda
Age (months) 1 23 4 (^689)
Body Length (in.) 8.0 11.75 15.5 16.7 20.1 22.2 *5 29.0
b. Reasoning Do you th in k you can use your m odel to extrapolate th e body
length of a 3-year-old p an d a? Explain.
338 Ch ap t er 5 Linear Funct ions