Dean Culver explains two methods for finding the center of a triangle in AutoCAD.
"I have found that there is usually more than one way to accomplish a task in AutoCAD. For example, I occasionally need to find the center or centroid of a triangle, and I've found two different ways to do so.
"First, I will use AutoCAD's internal calculator to place a point at the center of the triangle. I like to place a point there so I have something I can reference later if I need to dimension to it. Set the PDMode to the desired configuration (I prefer the setting of 3; that way, X marks the spot). Start by entering Point at the Command line, followed by the Enter key. It will then ask you to specify a point. Type in ‘CAL followed by the Enter key, which will activate AutoCAD's internal calculator (the apostrophe before the command allows it to be operated transparently while you are in another command). Next, AutoCAD will ask you for the >>>> expression: Type in the expression (INT+INT+INT)/3 followed by the Enter key. Your mouse pointer will be replaced with a pick box; carefully select the three intersections of the triangle. Although it may not appear to be picking anything, rest assured it is picking the intersections, provided your aim is good and they do indeed intersect. It will place a point at the center or centroid of the triangle.
"The second method to find the center of a triangle is to turn the triangle into a region. Start by entering Region at the Command line, followed by the Enter key. It will then ask you to select objects: pick the three sides of the triangle, if the triangle was made using lines, or the whole triangle if it is a pline, then use the Enter key to finish the command. Next use the MassProp command and select the newly created region. AutoCAD will list the x and y coordinates of the centroid of the triangle in the AutoCAD Text Window."
Notes from Cadalyst Tip Reviewer Brian Benton: This tip actually has five tips inside it: The first is that there is usually more than one way to do something in AutoCAD. The second and third are the tips on finding the center of a triangle. (A third method to find the center is to draw a line from one vertex to the midpoint of the opposite leg. Do this again on another vertex/leg. Where your two lines intersect will be the center. But that's a geometry tip, not a CAD tip.) A fourth tip is to use the Cal command inside an active command. And yet another tip is to use the MassProp command to find information about objects (regions, solids, etc.).