Creating a Right Triangle with "Polyline" Function
in AutoCAD 2009

 
Polyline Function in AutoCAD 2009, Image 1

Command: PLINE
Specify start point: 1000,1000

Specify next point or [Arc/Halfwidth/Length/Undo/Width] : 800

[Any number (for length of segment) would have done, or any point. To highlight the accuracy of CAD, I have entered in triangle side lengths (800 and 600) that would give an easily checked hypotenuse (1000).]
Polyline Function in AutoCAD 2009, Image 2

Keep at 0 degrees from X-axis.

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width] : 600
Polyline Function in AutoCAD 2009, Image 3

Move mouse to Endpoint. The length displayed should read 1000.

And it does.

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width] : C

This closes the triangle.
Polyline Function in AutoCAD 2009, Image 4

The completed right triangle.