Writing basic formulas

LaTeX knows three general modes:

• The paragraph mode: The text is typeset as a sequence of words in lines, paragraphs, and pages. That's what we used until now.

• The left-to-right mode: The text is also considered to be a sequence of words, but LaTeX typesets it from left to right without breaking the line. For instance, the argument of \mbox will be typeset in this mode; that's why \mbox prevents hyphenation.

• The math mode: Letters are treated as math symbols. That's why they're typeset in italic shape, which is common for variables. A lot of symbols can be used, most of them exclusively in this mode. Such symbols are roots, sum signs, relation signs, math accents, arrows, and various delimiters like brackets and braces. Space characters between letters and symbols are ignored. Instead, the spacing depends on the type of symbols—distances to relation signs are different from distances to opening or closing delimiters. This mode is required for all math expressions...