# Align equals inside case statement

I have the following code:

$$\frac {\partial C}{\partial {z_k}}= \begin{cases} - \frac {1} {y_i} {y_i} (1-{y_i}) = y_{i} - 1 & k = i \\ \frac {1} {y_i} {y_{i}^{2}} = y_{i} & k \ne i \\ \end{cases}$$


It works fine, but what I want to achieve is to align the = from the first case and the second case. I did not manage to do that.

Thank you!

I can be done, using aligned instead, but I am not sure which exact alignment is desirable. Probably the best is to use a second cases for the simplification.

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\frac {\partial C}{\partial {z_k}} = \left\{ \begin{aligned} - \frac {1} {y_i} {y_i} (1-{y_i})&= y_{i} - 1, &\qquad& k = i, \\ \frac {1} {y_i} {y_{i}^{2}}&= y_{i}, && k \ne i. \end{aligned} \right.

$$\begin{split} \frac {\partial C}{\partial {z_k}} &= \begin{dcases} - \frac {1} {y_i} {y_i} (1-{y_i}),& k = i, \\ \frac {1} {y_i} {y_{i}^{2}}, & k \ne i, \end{dcases}\\ &= \begin{cases} y_{i} - 1, &k = i, \\ y_{i}, & k \ne i. \end{cases} \end{split}$$

\end{document}

• Thank you for explaining and for giving me multiple options. I tried both (I was inclined to use the first one due in order to not take so much space) but I agree that the second one looks better, so I used that one. – elaRosca Jun 9 '14 at 10:56

It will suffice to insert \hspace{3.9em} at the beginning of your fourth line (the exact amount of space depends on the fontsize and the font you have chosen.

• Such manual adjustment is generally not advisable. – Andrew Swann Jun 9 '14 at 11:04