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In the below equation

$\Big(R - \left(\frac{D}{Q} \right) \Big) \Big( R - \left(\frac{D}{P} \right)\Big)$.

the \frac{D}{Q} and \frac{D}{P} have different sized parenthesis. However, if we use

$$\Big(R - \left(\frac{D}{Q} \right) \Big) \Big( R - \left(\frac{D}{P} \right)\Big)$$

they will have the same size.

What is the reason here and how we can solve the issue?

I'm using MathJax and tested the issue in LaTeX, too. When I put \displaystyle into the equation (with one dollar) it resolved but everything becomes bigger as double dollar.

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    $ places math in so-called \textstyle (for inline math), wheres $$ places math in \displaystyle, setting the equation apart from the text, and using larger notations in some cases. In LaTeX, $$ should be avoided. Use \[...\] instead or an equation environment, or one of the amsmath environments. – Steven B. Segletes Sep 23 '19 at 14:53
  • @StevenB.Segletes I've updated the question, I was tough this is only LaTeX related. – kelalaka Sep 23 '19 at 15:09
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    Anyawy, it's usually better to specify sizes with \Bigl ... \Bigr` and the like. This being said, the reason is certainly that the letter Q has a descender, which is taken into account to determine the parentheses size in the | \left ... right version. – Bernard Sep 23 '19 at 15:13
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    It may not be relevant here, but it's worth noting that the "sized" parentheses in different fonts aren't uniformly graded, so substituting font B for font A may get different results. – barbara beeton Sep 23 '19 at 15:20
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The difference is due to the fact that Q has a descendant while P doesn't, so in trying to make things tighter \left and \right are able to choose a smaller size.

You can solve it with

$\Big(R - \left(\frac{D}{Q} \right) \Big) \Big( R - \left(\frac{D}{\mathstrut P} \right)\Big)$.

or a \phantom{Q} --- at leat in LaTeX. Or, better, use \bigr etc as you are doing in the external brackets.

MathJax I don't know, and it's not on topic here --- the engine is very different.

|improve this answer|||||
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    This solved in MathJax, too. – kelalaka Sep 23 '19 at 15:16

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