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Math question? - September 9th 2009, 11:50 PM

Okay, I have a question and it might sound really stupid but my math teacher told me this and it kind of caught me off guard so I thought I would come on here and ask you all and see if any of you had heard this.

Okay so my class had this math problem and the end result was dividing two decimals.

5.1/3.1= 1.64

However, my math teacher said that we cannot divide two decimals because then then answer would be an estimate instead of an actual fact or whatever.

So, she said to write it like this:

5.1/3.1= 51/31

And that would be the answer. I have never heard of this before and the thing that makes it even more confusing is that the answer in the back of the book was 1.64.

Basically, my question is has anyone else heard of this rule or whatever? My teacher said if we don't do it this way we will get docked points and I just don't understand it. But, if other people have heard of it then I must just be behind in the times.

Thanks in advance for any replies.

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Re: Math question? - September 9th 2009, 11:52 PM

I've never heard of that and I'm taking 2 maths this semester. I've always converted the decimal(s) to a fraction(s) that way I get an actual number.

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Re: Math question? - September 10th 2009, 12:14 AM

I'm pretty sure it's sig fig stuff (significant figures), or at least it's somewhat similar. You deal with them mostly in science. It's about how like you can't be any more precise than the numbers given to you and weird stuff.

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Re: Math question? - September 10th 2009, 01:14 AM

This is something I have done in math for the past two years. My math teacher was very specific that the answers had to be in fractions, even if it didn't have them that way in the back of the book. This frustrated me, because checking my answers was made more difficult, but that's the way I had to do it.
So, long story short, I have heard of this, and I have to do math this way. Having the decimal is an approximate answer, and if you want your answer to be exact (which is very important in math) then use fractions.

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Re: Math question? - September 10th 2009, 03:22 AM

I've heard of it before and the reason is quite simple. With fractions, you can get an exact answer, whereas decimals cannot always give you an exact answer. Take for example 1/3, if you get a decimal answer, it's not accurate because it's not 1.333 nor is it 1.3333333333. It's an endless amount, which you can express as 1/3. Now, if it's something such as 1/2, then saying 0.5 is also giving the exact answer. Point is though, fractions always give you the exact answer in these cases whereas decimals don't always give you the exact answer.

But to answer your specific question, there is yet another reason. The reason has to do with significant figures, in that you are dividing decimals that have a significant figure in the tenths yet your answer would be in the hundredths. When you express it as a fraction, yes 51/31 would equal 1.64 but 5.1/3.1, if you're using significant figures, shouldn't be 1.64. With significant figures, the answer should be 1.6, however, we both know that the answer is not 1.6, it's 1.64.

If I were you, I'd go with the teacher. If you don't understand the reasons why, then here's the last and simplest reason: your teacher is doing the marking, not the book or its authors (unless of course your teacher is the author(s)).
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