cross-entropy代价函数是怎么推出来的
\begin{eqnarray}
\frac{\partial C}{\partial b}
= \frac{\partial C}{\partial a}\frac{\partial a}{\partial z}\frac{\partial z}{\partial b}
= \frac{\partial C}{\partial a} \sigma'(z).
\tag{73}\end{eqnarray}
以此反推出：
\begin{eqnarray}
\frac{\partial C}{\partial a} = \frac{a-y}{a(1-a)}
\tag{75}\end{eqnarray}
因此得出：
\begin{eqnarray}
C = -[y \ln a + (1-y) \ln (1-a)]+ {\rm constant},
\tag{76}\end{eqnarray}
[?]公式中的常数项是怎么去掉的？
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