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Math Help - Is PEMDAS wrong?!?

  1. #1
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    Is PEMDAS wrong?!?

    In my 8th grade math calss today, our teacher gave us this problem:
    5+4/8*9-8+6+5-8+6/24+21/12-5+5+3. According to our TI-30's, which follow PEMDAS, gave us. According to PEMDAS, you should solve the equation accordin go its rules. But thats not right ,because division and mutliplication do not out rank each other. So should PEMDAS be:

    P
    E
    M/D
    A/S

    because addition and subtraction dont out rank each other either. That would also mean to solve the equation left to right, because bothe pairs of operations are being used, which means even though our TI-30's follow PEMDAS, they are wrong. Can anyone help me?!?
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  2. #2
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    In 5+4/8*9-8+6+5-8+6/24+21/12-5+5+3 you must do the multiplications and divisions ahead of the additions and subtractions: that is, 5+(4/8*9)-8+6+5-8+(6/24)+(21/12)-5+5+3. You do the multiplications and divisions left-to-right so 4/8*9 is 4, divided by 8, result multiplied by 9 (that is, 4.5). You should get 5+4.5-8+6+5-8+0.25+1.75-5+5+3. Now do the additions and subtractions left-to-right, so 5, add 4.5, subtract 8 from total, add 6 to total, ...

    A few points to note. 1. PEMDAS (and its friends such as BODMAS) are conventions made up and agreed on by people to communicate mathematical formulae unambiguously with the minimum of effort and maximum of clarity -- see how tedious that formula would be if you had to bracket it fully: ((((((((((5+((4/8)*9))-8)+6)+5)-8)+(6/24))+(21/12))-5)+5)+3). As conventions they can't be right or wrong, merely in use or not in use in a particular context. 2. Not all calculators manage to follow the same conventions as each other, your teacher or your books; nor do they always follow them correctly. For that matter neither do all teachers or books ... 3. A minor point of terminology: you're not solving an equation here, you're computing the value of a formula. There's no equals sign anywhere.
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  3. #3
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    Thanks alot
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