Determine which two equations represent perpendicular lines.

Question

a) y=2x - 6
b) y=1/2x + 6
c) y= -1/2x + 6
d) y= 1/2x - 6

I know that perpendicular lines have negative recipricals, but I'm having trouble with this problem. I think the answer is b & c because they are both 1/2 and they have the (+6) and not (-6). I'm really confused though, because 1/2 and -1/2 aren't really negative recipricals because its the same number. I was thinking it should be 2 and -1/2.(which would make it perpendicular) I'm lost! does the plus 6 and minus 6 matter??

The SLOPES of perpendicular lines are negative reciprocals. That means the product of the two slopes is -1. Clearly, this applies to (a) and (c), for which the slopes are 2 and -1/2.