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4 9. The Sign of Continuation, $$\cdots$$, is read "and so on."

10. The Sign of Deduction, $$\therefore$$, is read "therefore" or "hence."

11. In arithmetical multiplication the order in which the factors of a product are written is immaterial. Thus

In like manner in Algebra $$ab$$ and $$ba$$ each denote the product of the two quantities represented by the letters $$a$$ and $$b$$, and have therefore the same value. Although it is immaterial in what order the factors of a product are written, it is usual to arrange them alphabetically. Fractional coefficients which are greater than unity are usually kept in the form of improper fractions.

12. When several different quantities are multiplied together a notation similar to that of Art. 7 is adopted. Thus $$aabbbbcddd$$ is written $$a^2b^4cd^3$$. And conversely $$7a^3cd^2$$ has the same meaning as $$7 \times a \times a \times a \times c \times d \times d$$.