function variance( a, b, c ) {
	return addon( a, b, c );
}
Returns the variance of a triangular distribution.

@private
@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, 5.0 );
// returns ~3.17

@example
var v = variance( -4.0, 4.0, 0.0 );
// returns ~2.67

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

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

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