It's not a mathematical thing, it's a problem with cutting fabric on the bias, and any triangle has to have at least two sides cut on the bias - not necessarily true bias which is 45 degrees to the grain, but nevertheless bias, or off-grain.
Things that will help are:
Choose a closely woven stable fabric.
Use starch.
Carefully stay stitch the edges - reduce presser foot tension and DON'T pull the fabric as you sew.
Press, don't iron.
Use a fusible interfacing.