实对称阵A是正定阵则A的特征值{a1,a2,..,an}都是正的而实对称阵是正交相似于对角阵diag(a1,..,an)即有正交阵P使得A=P'diag(a1,a2,..,an)P=P'diag(√a1,√a2,...,√an)·diag(√a1,√a2,...,√an)P记Q=diag(√a1,√a2...