[general_re]

1. The figure formed by two lines diverging from a common point.

A good illustration of the limits of general dictionaries in defining mathematical concepts. Consider triangles instead of squares for a moment. Given:

c² = a² + b² - 2abcos(X)

('X' only because I'm not sure everyone can see a theta in their browsers :^) )

What, then, is the value of the given angle 'X' when sides a, b, and c are all zero?

[BMCDA]

It's not defined.

[AndrewC]

What, then, is the value of the given angle 'X' when sides a, b, and c are all zero?

BMCDA answered you. I agree.