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Numeric and alphabetic expressions

A numerical expression is a combination of numbers, parentheses, and operation signs (++, -, \cdot, ÷\div). If there is no division by zero and the order of operations is followed (parentheses first, then multiplication and division, then addition and subtraction), you can evaluate it — you get a single number, the value of the expression. An algebraic (literal) expression adds letters to numbers and signs. Each letter stands for some number; different letters usually mean different numbers. Until numbers are chosen, there is no single “ready” value; after substituting numbers for letters you get a numerical expression you can evaluate. 5n5n or 5n5\cdot n is read “five times nn” — a product where nn is the second factor.
A numerical expression can be evaluated using order of operations
A letter is a placeholder for a number; substitute — get a numerical expression
The same letter in a problem usually stands for the same unknown
5n5\cdot n — multiply 5 by the value of nn
Value of a numerical expression: sum
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