Reflection: Developing a Conceptual Understanding Expressions Evaluated - Section 4: Closure

 

I anticipate that my students may say at any time during the class that they use the mnemonic "PEMDAS" or "Please Excuse My Dear Aunt Sally" to remind them of the order of operations to use when evaluating expressions with multiple operations. I tell students that I refuse to use “pendas” and suggest that they not use it either, unless they already know that it should not be strictly followed. Students of course will ask why??

I tell them that among other very good reasons for not using it, one reason is that PEMDAS, followed strictly can lead us to wrong answers, and I give them these examples.

1)      8 – 2 + 1

If we follow “pendas” we would do the addition before the subtraction and obtain

8 – 3 = 5   A math teacher would do 8 + (-2) + 1 which equals 7, the correct answer. This way it’s not ambiguous.

So what we really should do is add or subtract, whatever comes first from left to right.

2.  320 ÷ 4 x 20

Following “pendas” will give us an answer of 4, instead of the correct answer, 1,600.  We       multiply or divide, whatever comes first from left to right.

 


  Using PEMDAS?
  Developing a Conceptual Understanding: Using PEMDAS?
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Expressions Evaluated

Unit 1: Number Sense
Lesson 5 of 8

Objective: SWBAT evaluate numerical and algebraic expressions and recall important properties.

Big Idea: Understanding properties associated with evaluating expressions is essential for all further mathematics.

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Standards:
Subject(s):
Math, Numbers and Operations, Evaluating Expressions, order of operations
  55 minutes
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