# poorly sized square brackets due to equation over two lines -- how to equate their sizes?

Consider the following:

\documentclass[12pt,a4paper,oneside]{book}
\usepackage{amsfonts, graphicx, verbatim, mathtools,amssymb, amsthm, mathrsfs,amsmath}
\begin{document}
\begin{align*}
M_3&=\frac{(\mu(k)+\rho(k)+\xi(k))\left(\sigma_1^2(k)+2\int_M D_1^2(k,y)\nu(dy)\right)}{2M_1}S_k^{*^2}\\[1ex]    &+\frac{(\mu(k)+\alpha(k)+\theta(k)+\gamma(k))\left(\sigma_2^2(k)+2\int_M D_2^2(k,y)\nu(dy)\right)}{2M_2}I_k^{*^2}\\[1ex]
&+\frac{2\mu(k)+\rho(k)+\xi(k)+\alpha(k)+\theta(k)+\gamma(k)}{2\beta(k)(1-\rho(k))(1-\xi(k))}\left[\sigma_2^2(k)\right.\\
&\left.+2\int_M (D_2(k,y)-\log(1+D_2(k,y))\nu(dy)) \right]I_k^*
\end{align*}
\end{document}


How to i get the \left[ -- the "big" square bracket in line 3 -- to match the \right] big bracket in line 4?

• don't use \left\right use \bigl[ ... \bigr] or whatever size you need (this will be a duplicate) Commented Oct 27, 2022 at 13:10
• @DavidCarlisle This gives me an error..
– Math
Commented Oct 27, 2022 at 13:15
• then you did something wrong. You probably left in \right. or some such Commented Oct 27, 2022 at 13:22

To preserve the pairwise contents of the numerator terms of the three additive components of M_3, I would like to suggest you employ the \splitdfrac macro of the mathtools package for the numerator on line 3.

A separate issue: I think that the terms S_k^{*^2} and I_k^{*^2} in lines 1 and 2 are wrong, as the exponent 2 is too small. I believe {S_k^*}^2 and {I_k^*}^2 is better, as the exponent 2 will be rendered in \scriptstyle math mode instead of in \scriptscriptstyle math mode.

Note that there are no instances of \left and \right anywhere. That's quite deliberate.

\documentclass[12pt,a4paper,oneside]{book}
\usepackage{%amsfonts, %% amsfonts is loaded automatically by amssymb
graphicx, verbatim,
mathtools, % for '\splitdfrac' macro
amssymb, amsthm, mathrsfs,
%amsmath %% amsmath is loaded automatically by mathtools
}
\begin{document}
\begin{align*}
M_3
&=\frac{\bigl[\mu(k)+\rho(k)+\xi(k)\bigr]
\bigl[\sigma_1^2(k)+2\int_M D_1^2(k,y)\nu(dy)\bigr]}{%
2M_1} \, {S_k^*}^2 \\[1ex]
\frac{\bigl[\mu(k)+\alpha(k)+\theta(k)+\gamma(k)\bigr]
\bigl[\sigma_2^2(k)+2\int_M D_2^2(k,y)\nu(dy)\bigr]}{%
2M_2} \, {I_k^*}^2 \\[1ex]
\frac{\splitdfrac{\bigl[2\mu(k)+\rho(k)+\xi(k)
+\alpha(k)+\theta(k)+\gamma(k)\bigr]}
{\cdot\bigl[\sigma_2^2(k) +2\int_M
\bigl(D_2(k,y)-\log(1+D_2(k,y))\nu(dy) \bigr) \bigr]}}%
{2\beta(k)(1-\rho(k))(1-\xi(k))} \, I_k^* \\
\end{align*}
\end{document}

• This is nice but don't you think it is a little confusing compared to what i tried? I'm no expert(you are!) but that's my two cents :) Also, unrelated to this question(actually somewhat it is), I had asked David if he could "fix" the mistakes in my thesis but unfortunately he couldn't do it! Maybe can you? I will pay for your time :)
– Math
Commented Oct 27, 2022 at 16:22
• @Math - I guess it really depends on what you're looking to emphasize to the reader. My solution brings out the overall structure -- e.g., three additive terms, the numerators of the additive terms consists of a product of two components -- quite forcefully, at the cost of making the reader slow down a bit to take in what the numerator of the third term is all about. Put differently, readers may have to slow down a bit, but I don't think that the choice of notation will confuse them.
– Mico
Commented Oct 27, 2022 at 16:32
• Fair enough, I'll stick with your solution. So to my other question, would you be able to edit?
– Math
Commented Oct 27, 2022 at 16:42

Mico suggestion is the good one, but if you still want to use your coding as is, then try with the below:

Using left and right

\documentclass[12pt,a4paper,oneside]{book}
\usepackage{amsfonts, graphicx, verbatim, mathtools,amssymb, amsthm, mathrsfs,amsmath}
\begin{document}
\begin{align*}
M_3&=\frac{(\mu(k)+\rho(k)+\xi(k))\left(\sigma_1^2(k)+2\int_M D_1^2(k,y)\nu(dy)\right)}{2M_1}S_k^{*^2}\\[1ex]
\end{align*}


Using biggl and biggr

\begin{align*}
M_3&=\frac{(\mu(k)+\rho(k)+\xi(k))\left(\sigma_1^2(k)+2\int_M D_1^2(k,y)\nu(dy)\right)}{2M_1}S_k^{*^2}\\[1ex]

• You don't need \biggr. on line 3 or \biggl. on line 4. Furthermore, placing \biggl[\sigma_2^2(k) at the end of line 3 would appear to be inferior to placing it at the start of line 4..