CSDN uses KAtex and does not support align, but can use aligned.

$$\begin{aligned}
\frac{\partial l_n}{\partial w}&=-[\frac{y_n}{\sigma(z)}\sigma'(z)x - \frac{1-y_n}{1-\sigma(z)}\sigma'(z)x] \\
&= \frac{\sigma(z)-y_n}{\sigma(z)[1-\sigma(z)]}\sigma'(z)x \\
\end{aligned}$$

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\begin{aligned} \frac{\partial l}u n}{\partial w}&=-[\ frac{y}{\sigma(z)}\sigma'(z)x – \frac{y-y n}{1-\sigma(z)}\sigma'(z)x] \\\\\\sigma &= \sigma(z)} frac{\sigma(z)-y}{\sigma(z)[1-\sigma(z)}\sigma'(z)x \\\\end{aligned}

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