function variance( a, b, c ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		isnan( c ) ||
		!( a <= c && c <= b )
	) {
		return NaN;
	}
	return ( (a*a) + (b*b) + (c*c) - (a*b) - (a*c) - (b*c) ) / 18.0;
}
Returns the variance of a triangular distribution.

@param {number} a - minimum support
@param {number} b - maximum support
@param {number} c - mode
@returns {PositiveNumber} variance

@example
var v = variance( 0.0, 1.0, 0.5 );
// returns ~0.042

@example
var v = variance( 4.0, 12.0, 9.0 );
// returns ~2.722

@example
var v = variance( -4.0, 4.0, -1.0 );
// returns ~2.722

@example
var v = variance( 1.0, -0.1, 0.5 );
// returns NaN

@example
var v = variance( 0.0, 1.0, 2.0 );
// returns NaN

@example
var v = variance( NaN, 4.0, 2.0 );
// returns NaN

@example
var v = variance( 0.0, NaN, 2.0 );
// returns NaN

@example
var v = variance( 0.0, 4.0, NaN );
// returns NaN