public static double fact(String params) throws Exception{
		Double fact=Double.parseDouble(ParseExpression.parse(params));
		System.out.println("fact:"+fact);
		double result=1;
		if(fact<1){
			throw new Exception(ERROR_MESSAGE[FUCTION_PARAMS_ERROR]);
		}
		while(fact>=1){
			result*=fact--;
		}
		return result;
	}