xTAx=tr(AxxT) x^T A x = tr(A x x^T) xTAx=tr(AxxT)
xTAx=∑i,jai,jxixj x^T A x = \sum_{i,j} a_{i,j} x_i x_j xTAx=i,j∑ai,jxixj
xTAx=tr(AxxT) x^T A x = tr(A x x^T) xTAx=tr(AxxT)
xTAx=∑i,jai,jxixj x^T A x = \sum_{i,j} a_{i,j} x_i x_j xTAx=i,j∑ai,jxixj