# Preventing Certain Errors in Writing Formulas

You need to observe some rules to get a correct display of the formulas, and to prevent errors in writing systems, matrices, etc. NeoOffice Math points to these errors by displaying a red interrogation mark.

## Alignment of Various Elements in a Formula

To align correctly the elements of a formula or of equations, you may use the matrix, stack, binom, newline commands and

use the spaces which are writen ~ . If you want, you can put them in braces {~}.
The number of spaces which are to be added depends of the terms of the formula and, sometimes, you will have to proceed by trial and error to get a good result.

or

use the alignr et alignl commands.
You can replace alignl with quotes "", that you must place before the term which needs to be left aligned.

The examples below show these various methods and some differences of layout that they produce.

### Alignment of Several Equations

Note : In writing matrices, a +, – ou = sign, must be put in quotes, if it is an element of the matrix, i.e. if it is placed between two #.

Command Formula
left lbrace binom{a+b+c+d=f+g}{alignr h+k=i{~~~~} } right none
matrix {alignr a*(b+c)#"="#alignl ab+ac ## alignr (a+c)*(b+d)#"="#ab+cb+ad+cd}
left lbrace matrix{alignr x+2y+5z#"="# "" 3 ## alignr x + y~~~~~ #"="# "" -2## alignr x~~~~ "–" 3z #"="# "" 1} right none
matrix{ x+y+z#"="#""t ## alignr x+y#"="#""t–z## alignr x#"="#t-z-y } }
""x+y+z=t newline ""~~~x+y=t-z newline ~~~~~~~x=t-z-y

### Alignment of Addition, Substraction or Multiplication Signs

Command Formula
a+b+c{~~~} newline a-b+c-d
~~~~a*(b+c) newline (a+c)*(b+d)
a=\[b+c+g+h+j+i+k+l+m+dotslow newline ""{~~~~}d+f\]

## Avoiding Gaps

If you leave empty a location where Math is waiting for "something", you get an error message, i.e. a red quotation mark.

### Empty Boxes in Matrices

In a matrix, if you want that a box remains empty, you need to place a space ~, `, or {} in it.

Command Formula
left ( matrix{I_n # ~ ## ~ # I_m}right )=I_{n+m}
left ( matrix{A_n # {}# {} ## {} # dotsdown # {} ## {}#{}# A_m}right )

### Binar Operators

A term must be present on the two sides of the =, <, ≤, >, ≥, +, –, * signs, and of the union and intersection of sets symbols.

x+y={} et {}intersection _{i in I} %omega_i

Note – This last formula is not always satisfying. The intersection symbol is to small if the sets are named with capital letters. In order to fix that issue, a better method is described in this article in NeoWiki.