What is the correct order of operations? Why use it? What is PEMDAS?The order of operations in which one is to interpret a mathematical expression such asAnother way of saying this is that rather than being inherent in the structure of mathematics, the concept of "order of operations" is a matter of mathematical notation. Order of operations refers to which operations should be performed in what order, but it's just convention. The notation tells you which operations to do first, not something about the underlying mathematics. To remember the conventional order of operations, you can think of
PEMDAS
This means that you should do what is possible within parentheses first, then exponents, then multiplication and division (from left to right), and then addition and subtraction (from left to right). If parentheses are enclosed within other parentheses, work from the inside out.
^{1}Some people are taught to remember BEDMAS: Here are two examples:
3 + 5 x 7 = 3 + 35 = 38
(1 + 3) x (8  4) = ?
Logs, trig functions, and expressions involving eIn questions where order of operations must be considered, logs, trig functions, and expressions involving e are all treated as functions. This means that you have to evaluate them (turn them into numbers) before you can multiply, divide, add, or subtract. Before you can evaluate 6*f(4), you need to know the value of f(4) so that you can get a number for an answer. To get a number for an answer, you can only perform operations on numbers, so you have to evaluate all functions before you do anything with them.Examples:
The main thing that must be true in a notational system is consistency: when you use conventional notation you don't just start from the left and chug through, but you use the traditional order of operations. If you were to use another notational system, you would stick to that just as strictly.
The best reason for using conventional order of operations is the flexibility it gives you in writing down mathematical expressions. Remember that the operations of addition and multiplication are commutative, that is, they give you the same result no matter what order you write them in: 2*3 = 3*2. In conventional notation, that property is clearly reflected, and you get lots of options for how you want to write your expressions:
For more on different notation systems, see Order of Operations in Equations from the Dr. Math archives. To read an excellent discussion of a confusing question, see Mathematica Expressions vs. English; Calculators. More from the Dr. Math Archives:
Order of Algebraic Operations Order of Operations with Percentages Another Order of Operations Problem Order of Operations in Four Steps Order of Operations with Exponents Order of Operations and Fractions Order of Operations without Parentheses Precedence of Unary Operators Factorials and Order of Operations History of the Order of Operations Or search the Dr. Math archives for "order of operations" (just the words, not the quotes).

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