错误 1 使用了未赋值的局部变量“a1”

哪位大神指导下，感激不尽了
 static void Main(string[] args)
        {
            double a=5;
            double b=3;
            double a1;
            double a2;
            int cishu = 1;
            while (cishu <= 10)
            {
                if (cishu == 1)
                {
                    a1 = a * b / (b - a);
                }
                else
                {
                    a2 = a1 + b;
                }
                cishu = cishu + 1;
            }         
        }