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How to make \left, \right pairs of delimiter work over multiple lines?

Hi I'm trying to write a long equation with {align*}:

\begin{align*}
&\left|<L(w^2u),Lu>_k-<L(wu),L(wu)>_k \right|= \\
&|<L(w^2u),Lu>_k-<wL(wu),Lu>_k+<wL(wu),Lu>_k\\
&-<L(wu),wLu>_k+<L(wu),wLu>_k-<L(wu),L(wu)>_k|\\
&\leq \left|<(Lw-wL)(wu),Lu>_k\right|+\left|<wL(wu),Lu>_k-<L(wu),L(wu)>_k\right|\\
&+\left|<L(wu),(wL-Lw)u>_k\right|
\end{align*}

and I find that when I try to add \left| and \right to the second and third line the code doesn't compile. Can someone tell me what is wrong with this and how to fix it:

\begin{align*}
&\left|<L(w^2u),Lu>_k-<L(wu),L(wu)>_k \right|= \\
&\left|<L(w^2u),Lu>_k-<wL(wu),Lu>_k+<wL(wu),Lu>_k\\
&-<L(wu),wLu>_k+<L(wu),wLu>_k-<L(wu),L(wu)>_k\right|\\
&\leq \left|<(Lw-wL)(wu),Lu>_k\right|+\left|<wL(wu),Lu>_k-<L(wu),L(wu)>_k\right|\\
&+\left|<L(wu),(wL-Lw)u>_k\right|
\end{align*}
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marked as duplicate by Werner, Torbjørn T., Harish Kumar, egreg, zeroth Sep 12 '12 at 19:44

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A comment not related to pairing \left and \right directives, but about how to write the "angle" symbols around inner products. You shouldn't use < and >, in part because the spacing will be quite wrong. It's much better to use \langle and \rangle. –  Mico Sep 12 '12 at 20:48
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1 Answer

up vote 3 down vote accepted

\left can not live without \right at the same line. You may skip it by adding \right. or \left. which "closes" the line.

\begin{align*}
&\left|<L(w^2u),Lu>_k-<L(wu),L(wu)>_k \right|= \\
&\left|<L(w^2u),Lu>_k-<wL(wu),Lu>_k+<wL(wu),Lu>_k\right.\\
&\left.-<L(wu),wLu>_k+<L(wu),wLu>_k-<L(wu),L(wu)>_k\right|\\
&\leq \left|<(Lw-wL)(wu),Lu>_k\right|+\left|<wL(wu),Lu>_k-<L(wu),L(wu)>_k\right|\\
&+\left|<L(wu),(wL-Lw)u>_k\right|
\end{align*}
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