David Little
Mathematics Department
Penn State University
Eberly College of Science
University Park, PA 16802
Office: 403 McAllister
Phone: (814) 865-3329
Fax: (814) 865-3735
e-mail:dlittle@psu.edu

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Constructing A Perpendicular Line
Our goal here is to draw a line through the blue point and perpendicular to the line through the red and green points.

First, draw two circles, one centered at the red point and the other centered at the green point. Both circles should pass through the blue point.

Now, find the points of intersection of the two circles you just drew. Of course, one of these points is the blue point. And finally, draw a line through these two points of intersection.

You have just drawn line through the blue point that is perpendicular to the line through the red and green points. Can you explain why?