# Numbers and Equations

**Writing Numbers in Text**

In scientific writing, it is customary to write out numbers one through nine in the text. It is also customary to use Arabic numerals for numbers 10 and above, except when the number appears at the beginning of a sentence. Numbers appearing at the beginning of a sentence should always be written out. Other notable exceptions: Arabic numerals are always used with percent, units or actual measurements, time (when used with a.m. and p.m.), fractional numbers, and data taken directly from a table or figure.

**Equations**

Equations appear either as part of running text in a paragraph, or are set apart from the text (displayed), depending on their length. In either case, punctuation must be used appropriately. Equations must be typeset; handwritten characters are not acceptable.

### Placing equations

Equations may occur in running text, but all numbered equations must be displayed, that is, placed on separate lines and either centered or indented a consistent distance from the left text margin.Numbering equations

All equations referred to in the text must be numbered, although not all displayed equations must be numbered. As in numbering figures and tables, a double numbering system is used for equations; for example, Equation 2.1, where 2 is the chapter, and 1 is the first numbered equation in that chapter.

Γ − δ × a = 0 (2.1)

The equation number is placed flush with the right text margin and enclosed in parentheses. Equations in running text are not numbered.

### Breaking equations

An equation too long for one line is broken before an operational sign. Unless the equation is centered, the second line can be flush right, aligned on an operational sign, or indented far enough from the right to make room for the equation number. Equations longer than two lines are aligned on operational or descriptive signs. Equations may span multiple pages.

If a repeated part of the formula is replaced with an ellipsis (three spaced periods [. . .]), all appropriate punctuation and operational signs (for example, a final comma or sign in a series) are still included. Thus, the writer uses the following format:

P1, P2, . . . , Pn

P1 + P2 + . . . + Pn

P = 1, 2, . . .

not

P1, P2, . . . Pn

P1 + P2 + . . . Pn

P = 1, 2 . . .