Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Either I need my head examined or this is worng

  1. Jul 13, 2009 #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.
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jul 13, 2009 #2

    Borek

    User Avatar

    Staff: Mentor

    If you are getting -22 you have subtracted the result of multiplication instead of adding it.
     
  4. Jul 13, 2009 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    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.
     
    Last edited: Jul 13, 2009
  5. Jul 13, 2009 #4

    Borek

    User Avatar

    Staff: Mentor

    Wow, once I spotted the sign problem I was not digging deeper :grumpy:
     
  6. Jul 13, 2009 #5
    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.
     
  7. Jul 13, 2009 #6

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    I'm just better than you at making mistakes!:biggrin:
     
  8. Jul 18, 2009 #7
    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!
     
  9. Jul 21, 2009 #8

    I think we all must be going insane!

    For 3 plus 4 times 2

    I get 14 or 11
     
  10. Jul 23, 2009 #9
    looks likes gonna be 3 tough months
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Either I need my head examined or this is worng
Loading...