How to put a box around just one equation inside eqnarray?

Question

I would like to put a box around \dot{\mathcal{N}}_0+ik\mathcal{N}_1=-\dot{\Phi} inside the eqnarray below. I used \boxed{}, but did not work.

\documentclass[a4paper,12pt]{article}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage[makeroom]{cancel}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{graphicx,color,epsf}
\begin{document}
\begin{eqnarray}\nonumber
\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\dot{\mathcal{N}}+ik\int_{-1}^{1}\frac{d\mu}{2}\underset{P_1}{\underbrace{P_0(\mu)\mu}}\,\mathcal{N}&=&-\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\dot{\Phi}-ik\overset{\displaystyle =0}{\cancel{\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\mu\Psi}}\\
\nonumber
\dot{\mathcal{N}}_0+ik\mathcal{N}_1&=&-\dot{\Phi}
\end{eqnarray}

\end{document}

Answer 1

Don't use eqnarray. Use align and then mathtools instead of amsmath. Now you have \Aboxed macro. \boxed can't be used across &.

\documentclass[a4paper,12pt]{article}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage{amsfonts}
\usepackage{mathtools}
\usepackage[makeroom]{cancel}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{graphicx,color,epsf}
\begin{document}
\begin{align}\nonumber
\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\dot{\mathcal{N}}+ik\int_{-1}^{1}\frac{d\mu}{2}\underset{P_1}
{\underbrace{P_0(\mu)\mu}}\,\mathcal{N}&=-\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\dot{\Phi}
-ik\overset{\displaystyle =0}{\cancel{\int_{-1}^{1}\frac{d\mu}{2}P_0(\mu)\mu\Psi}}\\
\nonumber
\Aboxed{\dot{\mathcal{N}}_0+ik\mathcal{N}_1&=-\dot{\Phi}}
\end{align}

\end{document}