Either I need my head examined or this is worng

  • #1
http://library.thinkquest.org/10796/ch1/ch1.htm

I have some down time for the next 3 months and figured I might as well do something constructive and teach myself physics.

I am doing so on thee above website, but question 2 on section 3 has me stumped.

This section is about when and how to properly round, but I am baffled.

Question: -9.300 + 2.4 * 3.21

I am getting -22.149 which properly rounded would be -22, but they are saying it is -1.6

This just can't be right

If you put the wrong answer in enough times it will give you the correct answer.

Thank you in advance for all help.
 
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Answers and Replies

  • #2
Borek
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If you are getting -22 you have subtracted the result of multiplication instead of adding it.
 
  • #3
HallsofIvy
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Would you like us to suggest a good psychiatrist?

I finally got -22.149 but I had to make two mistakes.

1: replace the "+" by "-" as Borek suggests and
2: (far worse) not abide by the "order of operations" and do (-9.300- 2.4)*3.21.
 
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  • #4
Borek
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Wow, once I spotted the sign problem I was not digging deeper :grumpy:
 
  • #5
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You only need to make the "reordering the operations" mistake to get -22.149 since (-9.300+2.4)*3.21=-22.149. To sakartelephon, you cannot just do individual computations from left to right; you must first order your computations based on the type of operation. The order of operations, from first to last, is parentheses, exponents, multiplication/division, addition/subtraction (the standard mnemonic to remember this is "please excuse my dear Aunt Sally"); this is more thoroughly explained at http://en.wikipedia.org/wiki/Order_of_operations . For -9.300+2.4*3.21, we must first multiply 2.4 and 3.21 to get 7.70400, and then add -9.300 and 7.70400 to get -1.59600.
 
  • #6
HallsofIvy
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Wow, once I spotted the sign problem I was not digging deeper :grumpy:
I'm just better than you at making mistakes!:biggrin:
 
  • #7
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You did the addition before the multiplication.
Lots of beginning students do that.
3 plus 4 times 2 is either 14 or 24 depending on whether it is 7 times 2 or 3 times 8.

HallsofIvy's mistake is even worse, he should have got more negative than -30 !!
Borek was wrong also!
 
  • #8
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3 plus 4 times 2 is either 14 or 24 depending on whether it is 7 times 2 or 3 times 8.

I think we all must be going insane!

For 3 plus 4 times 2

I get 14 or 11
 
  • #9
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looks likes gonna be 3 tough months
 

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