Algebra 1 Common Core Student Edition, Grade 8-9

Why isn't the

y -in te r c e p t 2?

In slope-intercept form,

the y-intercept b is added

to the term mx. Instead

o f subtracting 2, you add

the opposite, - 2.

Id en tifyin g Slope an d y-ln tercep t

What are the slope and y-intercept of the graph of y = 5x - 2?

y = mx + b Use slo p e -in te rc e p t form.

'y-intercept

y = 5x + ( - 2 ) Think of y = 5x - 2 as y = 5x + (-2).

The slope is 5; th e y -in terc ep t is - 2.

K& Got It? 1. a. W hat are th e slope an d y -in te rc e p t of th e graph of y = -\x + §?

b. Reasoning H ow do th e g raph of th e line a n d th e eq u a tio n in p a rt (a)

change if th e y -intercept is m oved dow n 3 units?

When can you use
slope-intercept form?
You can w rite an equation
o f a nonvertical line in
slope-intercept form if
you know its slope and
y-intercept.

I W r i t i n g a n E q u a t i o n i n S l o p e - i n t e r c e p t F o r m

What is an equation of the line with slope — | and y-intercept 7?

y = mx + b Use s lo p e -in te rc e p t form.

y = ~ 5 * + 7

An e q u a tio n for th e line is y

Substitute — jr for m and 7 fo r b.

r+ 7.

& Got It? 2. W hat is a n eq u a tio n of th e line w ith slope § an d y -intercept - 1?

What does the graph
tell you about the
slope?
Since the line slants up
from left to right, the
slope o f the line should
be positive. The line is
also fairly steep, so the
slope o f the line should
be greater than 1.

Writing an Equation From a Graph

Multiple Choice Which equation represents the line shown?

C3D y = ~2x + 1 CO y = |x — 2

CO y= 2x+ l C O y = 2x - 2

Find th e slope. Two points on the line are (0, - 2 ) an d (2, 2).

2 - ( - 2 ) 4

slope = 0 n = o = 22-0

The y-intercept is - 2. W rite a n eq u a tio n in slope-intercept form.

y = mx + b

y = 2x + ( - 2 ) S ub stitu te 2 for m and - 2 for b.

An equation for th e line is y = 2x — 2. The correct answ er is D.

Go t I t? 3. a. W hat do you expect th e slope of th e line to be from looking

at th e graph? Explain.

b. W hat is a n e q u a tio n of th e line show n at th e right?

c. Reasoning Does th e eq u a tio n of th e line d e p e n d on the

points you use to find th e slope? Explain.

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PowerAlgebra.com | L e s s o n 5 - 3 S l o p e - i n t e r c e p t F o r m 309