KaTeX parse error: No such environment: align
csdn 使用的是 KaTeX，不支持 align，但可以用 aligned。
$$\begin{aligned}
\frac{\partial l_n}{\partial w}&=-[\frac{y_n}{\sigma(z)}\sigma'(z)x - \frac{1-y_n}{1-\sigma(z)}\sigma'(z)x] \\
&= \frac{\sigma(z)-y_n}{\sigma(z)[1-\sigma(z)]}\sigma'(z)x \\
\end{aligned}$$
∂ l n ∂ w = − [ y n σ ( z ) σ ′ ( z ) x − 1 − y n 1 − σ ( z ) σ ′ ( z ) x ] = σ ( z ) − y n σ ( z ) [ 1 − σ ( z ) ] σ ′ ( z ) x \begin{aligned} \frac{\partial l_n}{\partial w}&=-[\frac{y_n}{\sigma(z)}\sigma'(z)x - \frac{1-y_n}{1-\sigma(z)}\sigma'(z)x] \\ &= \frac{\sigma(z)-y_n}{\sigma(z)[1-\sigma(z)]}\sigma'(z)x \\ \end{aligned} ∂w∂ln=−[σ(z)ynσ′(z)x−1−σ(z)1−ynσ′(z)x]=σ(z)[1−σ(z)]σ(z)−ynσ′(z)x