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# Perpendicularity condition Watch

1. I was wondering if there is anyone who can explain the following defintion I have of perpendicularity condition in a way that I can understand. In my text book it reads:

perpendicularity condition:
If two lines are perpendicular, then either the product of their slopes is -1 or one has slope 0 and the other has inficine slope.

Busy with circles at the moment. I've worked out the segment of the line and my answer is 3. But using the perpendicularity condition, the slope is therefore -1/3.

I'm grateful for any help.

Thanks
JB
2. (Original post by JB01)
I was wondering if there is anyone who can explain the following defintion I have of perpendicularity condition in a way that I can understand. In my text book it reads:

perpendicularity condition:
If two lines are perpendicular, then either the product of their slopes is -1 or one has slope 0 and the other has inficine slope.

Busy with circles at the moment. I've worked out the segment of the line and my answer is 3. But using the perpendicularity condition, the slope is therefore -1/3.

I'm grateful for any help.

Thanks
JB
Do you know how to work out the tangent of the angle between 2 straight lines in terms of the gradients of those lines? If you've seen that demonstrated, then you can see that as the angle between the lines approaches 90 degrees, then the product of their gradients must approach -1.

Edit: oh, and the condition about 0 and infinity is to deal with the special case where one of the lines is horizontal and the other vertical!

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