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09142011  #1 
Join Date: Sep 2011
Posts: 2

Order of operations and variables
best strategy to explain how to solve the following problem:
12^2 / 4b + 4 when b=9 Some students evaluated teh problem and got 8 while others got 338. 
09152011  #2 
Join Date: Mar 2005
Posts: 10,609

Corrected. Thank you jcsites and MAS1.
Substitute the value of 9 for b: Evaluate exponents: Divide: Multiply: Add:
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Do Math and you can do Anything! Last edited by Mr. Hui; 09242011 at 09:46 PM.. 
09212011  #3 
Guest
Posts: n/a

328
Pemdas
12 ^ 2 / 4 * 9 + 4 144 / 4 * 9 + 4 36 * 9 + 4 324 + 4 328 Last edited by Jonathan W; 12312011 at 07:32 PM.. Reason: Huge Mistake 
09222011  #4 
Join Date: Sep 2011
Posts: 5

It should be 328. The precedence of operation is as follows: exponentiation, division, multiplication and addition. Some got 8 because they first multiplied 4 with 9. However, since division and multiplication are of the same precedence, division is evaluated first.

09232011  #5 
Join Date: Sep 2011
Posts: 2

Rules that apply?
Edit: Please check the correction in post #2
That is the rule that states you do 4b before dividing 144/4? Anything about solving teh variable expressions first. Even the responses on this question are not the same. I appreciate any feedback. Last edited by Mr. Hui; 09242011 at 09:43 PM.. 
09242011  #6 
Join Date: Mar 2005
Posts: 10,609

Edit: Please check the correction in post #2
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Do Math and you can do Anything! Last edited by Mr. Hui; 09242011 at 09:43 PM.. 
09242011  #7  
Join Date: Dec 2008
Posts: 249

Quote:
Mr. Hui is assuming that the above expression, as written, is equivalent to: 12^2 / (4b) + 4 so that when b = 9 the answer is 4. When he says it doesn't matter what order you evaluate 144/4 or 4b first he is assuming you would get either 144/4/9, or 144/36 which are the same. However, others are assuming that the expression is equivalent to: (12^2 / 4) x b + 4 which evaluates to 328. As the expression is originally written I believe it has to be evaluated the second way so that the answer is 328. 12^2 / 4b + 4 144 / 4b + 4 Evaluate exponents 36 x b + 4 Evaluate division left to right 36 x 9 + 4 Substitution 324 + 4 Multiplication 328 Addition The 4b term is what is confusing as we are used to treating it as a term in itself so that it seems like we have 144 / 36 when b = 9. 

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