Which of the following pairs of lines are perpendicular?
a.y=2x-3 and y=(1/2)x+12
b.y=2x+5 and y=-2x-9
c.y=-2x-6 and y=(-1/2)x-6
d.y=2x-7 and y=(-1/2)x+87
I was thinking maybe it is B but I don't know. PLEASE HELP! and explain how you got it. Thanks

Question

chriss · Answer

It is c.  The slopes are reciprocals of each other and they share the same y-intercept.

anonymous · Answer

Is it because -2 can be 2/1?

chriss · Answer

Yes... except it would be -2/1, and that's why they are reciprocals... a reciprocal of one number is just the same fraction turned upside down.  So -2/1 is a reciprocal of -1/2 and vice versa.

anonymous · Answer

It's c.

Lines are perpendicular IF and ONLY IF their slopes are reciprocal negatives of each other, example say one line has the following slope:
$$m=\frac{a}{b}$$
then any line perpendicular to it will have the follwing slope

$$m_{2}= -\frac{b}{a}$$

anonymous · Answer

Than you

chriss · Answer

Right, I mentioned the y-intercept, but that actually isn't a requirement, but the slope reciprocal is.

anonymous · Answer

It's all crystal!!!