正定矩阵A的特征值都是正的,可相似对角化成diag(a1,a2,...,an),ai>0.即存在正交矩阵P,使P'AP=diag(a1,a2,...,an)取C=diag(1/√a1,1/√a2,...,1/√an)则有C'P'APC=C'diag(a1,a2,...,an)C=E即(PC)'A(PC)=E