# Breaking lines in equations

I have an equation that is too long. It gets cut off, so I have tried to modify it with the split environment inside an equation environment:

$$\begin{split} & I_{ribbon} = \frac{1}{2} M(a^2 +b^2) \\ &= \frac{1 }{2}\times3.8\times0.001kg\times\Big(\frac{12.49\times0.001m}{2} + \frac{20.79\times0.001m}{2}\Big)^2 = 2.794\times10^{-7} kg\cdot m^2 \end{split}$$


The only issue now is that the equation number is on the line between the two lines of the equation, i.e., vertically centered across the two lines, rather that on the last line where I think it should be.

How can I fix this?

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Welcome to TeX.SX. A tip: If you indent lines by 4 spaces, then they're marked as a code sample. You can also highlight the code and click the "code" button ({}) or hit Ctrl+K. – Claudio Fiandrino Apr 7 '13 at 8:01

You can use the following:

\documentclass{article}

\usepackage[
tbtags
]{amsmath}
\usepackage[
product-units = single,
output-product = \times,
inter-unit-product = \cdot
]{siunitx}

\begin{document}

$$\begin{split} I_{\textup{ribbon}} &= \tfrac{1}{2} M(a^{2} + b^{2})\\ &= \tfrac{1}{2} \times \num{3.8} \times \SI{0.001}{\kg} \times \left(\frac{\num{12.49} \times \SI{0.001}{\mm}}{2} + \frac{\num{20.79} \times \SI{0.001}{\m}}{2}\right)^{2}\\ &= \SI{2.794e-7}{\kg\square\m} \end{split}$$

\end{document}


P.S. When typesetting physical quantities, the siunitx package is the way to go.

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I'd rather use split inside an equation environment, that would place only one equation number taking into account options such as leqno, reqno, centertags or tbtags. – egreg Apr 7 '13 at 9:53
\text is not really a good idea for "ribbon", as far as I know it is not all constructions that can actually suffice with _\text{}..., in the just for the archives adding extra {} might be a good idea. Also at least on the first line \tfrac would look better – daleif Oct 31 '13 at 9:20
Probably using \textup, I_{\textup{ribbon}}. Would of course make a macro if used often. \textup has the advantage that things like æøå then works. \text is a bad idea if the context is italic – daleif Oct 31 '13 at 17:59