2.2.2 Equations of Lines - Part II
Horizontal and vertical lines
Horizontal lines:
y=a
passes through (0,a)
Vertical lines:
x=a
passes through (a,0)
(note that vertical lines are not functions)
Direct variation
 if a linear function is given by y = kx (or f(x) = kx)
(note that y-intercept is the origin)
 we say that y varies directly as x, or
 y is directly proportional to x
 with k as the constant of variation (or proportionality)
 notice that k is also the slope or rate of change
Example:
The electrical resistance of a wire varies directly with its
length. If a 255-foot length of wire has a resistance of 1.2
ohms, find the resistance of 135 feet of the same wire.
1. Write the equation: R = kL
2. Find the constant of prop.: 1.2 = k255  k = .00471
3. Answer the question: R = .00471(135) = .635 ohms
Interpret the constant of proportionality:
For this wire, the resistance is k = .00471 ohms/foot: the
resistance increases by .00471 ohms for each foot of
length.
2.2.2-1