1
$二次型导数:\frac{d(x^TAx)}{dx} = (A+A^T)x$
$且当A为对称矩阵时:A = A^T , \frac{d(x^TAx)}{dx} = (A+A^T)x = 2Ax$
2
$ \frac{d(A^Tx)}{dx} = \frac{d(x^TA)}{dx} = A$
3

$u,v,x:n*1$
$ \frac{dU^TV}{dx} = \frac{\partial U}{\partial x}v+\frac{\partial V}{\partial x}U$
4
$ \frac{d(U+V)}{dx} = \frac{dU}{dx}+\frac{dV}{dx}$

分子分母布局，分母横向拉伸，分子纵向拉伸