求两个向量的协方差

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根据D（A＋B）＝D（A）＋D（B）+2COV（A，B）计算

private double GetConv(double[] a, double[] b)        {            if (a.Length != b.Length)            {                return 0;//error            }            double cov = 0;            double meana = 0;            double meanb = 0;            double meanab = 0;            double detaa = 0;            double detab = 0;            double detaab = 0;            for (int i = 0; i < a.Length; i++)            {                meana += a[i];                meanb += b[i];                meanab += a[i]+ b[i];            }            meana /= a.Length;            meanb /= a.Length;            meanab /= a.Length;            for (int i = 0; i < a.Length; i++)            {                detaa += Math.Pow(a[i] - meana, 2);                detab += Math.Pow(b[i] - meanb, 2);                detaab += Math.Pow(a[i] + b[i] - meanab, 2);            }            cov= (detaab - detaa - detab) / 2;            return cov;        }