Algebra Period 3 (Friday)

Parallel and Perpendicular Lines 7-8

2 SPECIAL LINES AND THEIR SLOPES

PARALLEL LINES:
2 lines that are parallel to each have the SAME SLOPE!
y = 2x - 10 and y = 2x + 3/4 are parallel because they both have a slope of 2

PERPENDICULAR LINES:
2 lines that are perpendicular to each other have SLOPES that are:
OPPOSITE SIGNS
and
RECIPROCALS
y = 2x - 10 is perpendicular to y = -1/2 x + 3/4


THIS IS ANOTHER TWIST TO OUR MYSTERY LINE PUZZLE!!!
If you know that the mystery line is parallel or perpendicular to another given line
then you know the mystery line's slope!!!


EXAMPLE:
Your mystery line has a point of (2, -5) and
is PARALLEL to line y = 2x + 3/4
So you know the mystery line's slope because it is the same as the given line
( m = 2)
Substitute the slope and the point given on the mystery line and solve for b.


EXAMPLE:
Your mystery line has a point of (2, -5) and
is PERPENDICULAR to line y = 2x + 3/4
So you know the mystery line's slope because it is the opposite sign reciprocal of the given line

Lines that parallel to the y axis in the form x = c, ( such as x = 3) have an "undefined slope"
or NO SLOPE because if you find their slope using
y₂ - y₁ / x₂ - x₁
you will discover
a number/0
which is undefined

On the other had lines parallel to the x axis, in the form y = b ( such as y = -5) have a zero slope becuase using the same
y₂ - y₁ / x₂ - x₁
you will discover
0/ a number
which = 0