21.(本题满分12分)-|||-设3阶实对称矩阵 =((a)_(1),(a)_(2),(a)_(3)) ^2=A 且 (A)=2, _(1)+(a)_(2)=(a)_(3).-|||-(1)求矩阵A;-|||-(2)求正交矩阵Q,使得二次型x^TAx经正交变换 x=Qy 化为标准形.