When I enter 3xy the number 3 appears in italics. Why?
Math uses multi-character variable names, which is common practice in many disciplines such as economics. The rule is that each variable name consists of a sequence of letters, numbers and dots (the first character may not be a dot).
Thus, if you enter
3xy, it will be considered as a variable name, and variables are printed in italics. However strange this may seem, a variable name can begin with a digit!
The correct way of entering the expression for "3 times x times y" is
3 x y (with spaces between the characters) or better still
3 cdot x cdot y.