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Math like $x_1^2+\cdots x_n^2=\sum_{i=1}^{n} x_i$
can also be written as a displayed formula.
If the formula is presented as a display, like this one:
$$ x_1^2+\cdots x_n^2= \sum_{i=1}^{n} x_i\eqno(12345)$$
it is centered and the amount of white space arround it is adjusted to ``look good''.
It can also have equation number at its left side.
$$ x_1^2+\cdots x_n^2= \sum_{i=1}^{n} x_i\leqno(12345)$$
This equation
$$A = B\eqno(5)$$
is not very difficult.
It does'nt make it simpler if I use the commutative law and write it (with an
enourmous equation number to illustrate centering) like this:
$$B = A\eqno(500000000000000)$$
The space around a displayed equation depends on the length of the last line in
the paragraph preceeding it.
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