Misaligned "Aboxed" block and incorrect text size

Question

In the code below, my aboxed block is misaligned and also of a different size font than the normal text. Can you help me re-align the aboxed block with the above line and also correct the size of the text?

Here is the code:

\documentclass[11pt,fleqn]{book}

\usepackage{xcolor}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage[framemethod=default]{mdframed}
\usepackage{mathtools}
\usepackage[most]{tcolorbox}
\tcbset{myformula/.style={
  colback=blue!15,
  colframe=blue!65,
  }
}
\newenvironment{spread}[1]{%
  \advance\jot#1% indeed
  }{%
\ignorespacesafterend
}

\makeatletter
\patchcmd{\@Aboxed}{\boxed{#1#2}}{\colorbox{red!15}{$#1#2$}}{}{}%
\makeatother

\begin{document}

Solving for $Y_{3}$, we have
\begin{spread}{5pt}
\begin{tcolorbox}[ams align,myformula]
Y_{3}&=\frac{\dfrac{1+sC_{2}R_{2}}{R_{1}R_{2}}}{\dfrac{1+sC_{2}R_{2}}{R_{2}}+\dfrac{1}{R_{1}}+sC_{3}}\nonumber\\
&=\Aboxed{\frac{1+sC_{2}R_{2}}{R_{1}+sC_{2}R_{1}R_{2}+R_{2}+sC_{3}R_{1}R_{2}}}
\end{tcolorbox}
\end{spread}

\end{document} 

Answer 1

The intended use of \Aboxed is \Aboxed{x &= y}, i.e. making a box that crosses an alignment. Use it like this and the alignment is correct, though the equals sign of course is included in the box as well. To get the size correct, add \displaystyle in the patch, cf. Show inline math as if it were display math.

Also, you're reinventing the wheel with the spread environment, mathtools already defines the spreadlines environment, which does a similar job. (Only setting \jot instead of adding to it.)

\documentclass[11pt,fleqn]{book}

\usepackage{xcolor}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage[framemethod=default]{mdframed}
\usepackage{mathtools}
\usepackage[most]{tcolorbox}
\tcbset{myformula/.style={
  colback=blue!15,
  colframe=blue!65,
  }
}

\makeatletter
\patchcmd{\@Aboxed}{\boxed{#1#2}}{\colorbox{red!15}{$\displaystyle #1#2$}}{}{}%
\makeatother

\begin{document}

Solving for $Y_{3}$, we have
\begin{spreadlines}{8pt}
\begin{tcolorbox}[ams align,myformula]
Y_{3}&=\frac{\dfrac{1+sC_{2}R_{2}}{R_{1}R_{2}}}{\dfrac{1+sC_{2}R_{2}}{R_{2}}+\dfrac{1}{R_{1}}+sC_{3}}\nonumber\\
\Aboxed{&=\frac{1+sC_{2}R_{2}}{R_{1}+sC_{2}R_{1}R_{2}+R_{2}+sC_{3}R_{1}R_{2}}}
\end{tcolorbox}
\end{spreadlines}

\end{document} 

Answer 2

I know I'm late to this solution, but I had the same problem and found this question already asked on stackexchange. Although it does not include a fancy color box, my solution to the misalignment problem is to add an empty group to the left hand side of the \Aboxed command.

Here's a MWE:

\documentclass[]{article}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{align*}
\frac{d}{dx} (2+x)e^{-x^2} &= e^{-x^2}\frac{d}{dx} (2+x) + (2+x)\frac{d}{dx} e^{-x^2} & \text{(Product Rule)}\\
\Aboxed{{} &=-(2x^2+4x-1)e^{-x^2}}
\end{align*}
\end{document}

Since \Aboxed expects its input to be of the form A &= B, putting an empty group is enough to satisfy the requirements.