function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= a ) {
			return 0.0;
		}
		// Case: x > a
		if ( x <= c ) {
			return pow( x - a, 2.0 ) / denom1;
		}
		// Case: x > c
		if ( x < b ) {
			return 1 - ( pow( b - x, 2.0 ) / denom2 );
		}
		// Case: x >= b
		return 1.0;
	}
Evaluates the cumulative distribution function (CDF) for a triangular distribution.

@private
@param {number} x - input value
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0 );
// returns <number>