28.求一个正交变换化下列二次型成标准形:-|||-(1) =2({x)_(1)}^2+3({x)_(2)}^2+3({x)_(3)}^2+4(x)_(2)(x)_(3) 。-|||-(2) =({x)_(1)}^2+({x)_(3)}^2+2(x)_(1)(x)_(2)-2(x)_(2)(x)_(3)