Posted by Kendra on Thursday, August 26, 2010 at 10:42pm.
Write the equation of the line that passes through the (2,1) and is perpendicular to y=5.
Okay, I don't understand how I can do this without a slope.

Algebra  Reiny, Thursday, August 26, 2010 at 11:17pm
y = 5 or
y = 0x + 5 , has a slope of zero and is a horizontal line
so the perpendicular must be a vertical line, but
you probably have learned that a vertical line has an undefined slope.
It helps to know that any horizontal line has the form
y = k, where k is the y value of all the points on the horizontal line (they would all be the same)
the equation of a vertical line is x = k, were k is the x value of all the points on that vertical line.
so all we have to do is "look at" the given point,
and we see that the x value is 2
so the equation is simply
x = 2