1. Does y vary directly with x ? If
so, what is the constant of
variation, and the function rule?
2. y varies directly with
x, and y = 24 when
x = -15, what is x
when y = 16
1. k =
𝑦
𝑥
5
2
= =
12.5
5
=
25
10
= 2.5
Yes, y varies directly with x.
The constant of variation k is 2.5
y = kx
y = 2.5x
2. Suppose y varies directly with x,
and y = 24, when x = -15. What is
x when y = 16.
𝑦1
𝑥1
=
𝑦2
𝑥2
24

−15
=
16
𝑥
24x = -15(16)
24x = -240
24
24
x =-10
Wed
9/23
Learning Objective:
To write eq’ns of lines in
slope-intercept form
Lesson Hw: Pg. 78 # 13 – 27 odd,
2-3
47 – 49, CC *7, *46, *58
Algebra II
 To
write equations of lines in
slope-intercept form
 Linear
Equation – represents a
linear function.
y = 3x – 7
Solution of a linear equation – any
ordered pair (x, y) that makes the
equation true.
 Slope-intercept
form – an
equation of a line
y = mx + b
y-intercept
slope
y-intercept – point where line
crosses y-axis
x-intercept – point where line
crosses x-axis
1.
1
m=
5
and y-intercept is (0, -3)
y = 𝑚𝑥 + 𝑏
y=
1
𝑥
5
−3
2. m = 6 and y-intercept is (0, 5)
y = 𝑚𝑥 + 𝑏
y = 6x + 5
To rewrite a linear
equation into slopeintercept form
Get the equation into
y=𝑚𝑥+𝑏
by solving for y.
3.
5x – 4y = 16
-5x
-5x
-4y = -5x + 16
-4
-4
-4
y=
5
𝑥
4
−4
Slope =
5
4
y-int: (0, -4)
4.
3x + 2y = 18
-3x
-3x
2y = -3x + 18
2
2
2
y=
3
− 𝑥
2
Slope =
+9
3
−
2
y-int: (0, 9)
5.
3
(4)(− 𝑥
4
3
− 𝑥
4
+
+
1
𝑦)
2
1
𝑦
2
= −1
= −1(4) 
12
− 𝑥
4
+
4
𝑦
2
= −4
-3x + 2y = -4
+3x
+3x
2y = 3x – 4
2
2
2
3
y = 𝑥−2
2
Slope =
3
2
y-int: (0, -2)
6. Line 1: (2, 4) & (4, 2)
𝒚𝟐 − 𝒚𝟏 𝟐 − 𝟒 −𝟐
𝑚=
=
=
= −𝟏
𝒙𝟐 − 𝒙𝟏 𝟒 − 𝟐
𝟐
Line 2: (-1, -3) & (3, 1)
𝒚𝟐 − 𝒚𝟏 𝟏 − (−𝟑) 𝟒
𝑚=
=
= =𝟏
𝒙𝟐 − 𝒙𝟏 𝟑 − (−𝟏) 𝟒
Are the lines parallel, perpendicular, or neither? Why?
They are
perpendicular because
the slopes 1 and -1
are opposite
reciprocals!
y=12 + 10x models the amount of
money in your wallet.
y = total in dollars
x = # of weeks from today
1. Is it in slope intercept form?
2. What is the slope?
3. What does the slope represent?