At the risk of putting damper on the party ...

You did say "ACCURATELY". One of my major peeves with Xara is the ACCURACY thing. The click and drag method does work, but only by eye - the same as most of Xara's accuracy. It's not strictly accurate, as not only will Xara not constrain the drag of the triangle sides to EXACTLY perpendicular, but as far as I'm aware, you cannot pick the exact centre of the relevant side.

So athough it looks visually correct, it's not accurate.