
#1
Dec2912, 02:41 AM

P: 41

How do people end with .9r?
Apart from trying to derive square root of numbers using Newton's method, the biggest culprit is how we employ the authorised order of operation in mathematics i.e. BODMAS, PENDMAS e.t.c. Specificaly where we place the quantities. I saw an equation being used to 'reduce' .9r into 1 (or is it to collapse it into 1?) that went thus x=.9r 10x=9.9r subtracting the first equation from the second and we get 9x=9 dividing each side by 9 we find x=1 proving .9r is actually (equal to) 1. Here is the problem. If we decide to start with division before multiplication, we end up in the same hole that we're trying to get ourselves out of. Check this. (1/9)x9=? Now lets divide 1 by 9, what do we end up with? .111 . . . .i.e. 0.1r multiply that by 9 and we end up with .9r If we could only have rearranged the equation to be 1x(9/9) That way we will avoid unnecessary controversies (but again just like in tabloids, maths seems to thrive in controversies). 



#2
Dec2912, 03:11 AM

Sci Advisor
P: 779

I think what you are trying to say is:
##1 = \frac{9}{9} = \frac{1 \times 9}{9} = \frac{1}{9} \times 9 = 0.\overline{1} \times 9 = 0.\overline{9}## Of course when students see this they ask "but 1 divided by 9 isn't really 0.1 recurring, it's just an approximation" and we are back to square one. 



#3
Dec2912, 03:13 AM

P: 784

First of all. .9999.... is = to 1.
Second of all, these 'absurdities' like PEMDAS are crucial to mathematics. There are the rules math follows. To say they are arbitrary and wrong is like saying the dictionary definition for the word 'the' is wrong because 'the' can mean anything you want. Sure, you're right in a sense. But you're right in a useless way which gets you no where. We define an order of operations so that our math works. Without it, math can do nothing. Remember, all of math is made up, so we can define whatever we want (as long as the system is consistent). It's just a testament to the power of the things we 'define arbitrarily' that we can use math to so accurately describe the real world. 



#4
Dec2912, 03:43 AM

Sci Advisor
P: 779

Avoiding division absurdities.Just look at any nonassociative algebraic structure. Yes it annoying in that we need to write down the parenthesis, but that's it. It's an annoyance. 



#5
Dec2912, 05:55 AM

Sci Advisor
HW Helper
PF Gold
P: 12,016

Auxiliary rules are meant to..auxiliate, not have independent insights. We MIGHT write 3+4*5+5*(2+1) as A(A(3,M(4,5)),M(5,A(2,1)), where A(a,b) is the addition operator, and M(a,b) the multiplication operator, but it ought to visually intuitive why we do NOT, usually, write A(A(3,M(4,5)),M(5,A(2,1)) 



#6
Dec2912, 12:05 PM

P: 784

I understand your point, of course, but I don't agree with the conclusion. I do believe I know less math than the two of you, but is there a good reason ##3 \times 4 +5## implies ##(3 \times 4) + 5## rather than ##3 \times (4 + 5)##?
Isn't it that we've decided that with a lack of parenthesis multiplication goes first? Based on that, if that is correct, I would say something like PEMDAS is very much a rule. 



#7
Dec2912, 01:16 PM

P: 41

We wouldn't be in this quagmire if the Indians or Arabs didn't invent the decimal fraction. I guess we have to live with it.




#8
Dec2912, 01:37 PM

P: 771





#9
Dec2912, 01:45 PM

Mentor
P: 16,692





#10
Dec2912, 04:03 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,896





#11
Dec2912, 06:36 PM

P: 308

PEMDAS is a convention. There are controversies in math. These however aren't among them. 



#12
Jan513, 12:33 AM

P: 41





#14
Jan513, 03:35 AM

P: 642

Yep, if we divide by 9 instead of multiplying by 10, we end up deeper in the same hole.
But why do that if we can easily grab the vine of multiplying by 10, and then we just need to pull to get out of the hole? 


Register to reply 
Related Discussions  
Difficulty of lower division courses vs. upper division (undergraduate)  Academic Guidance  10  
Synthetic division or Long division of polynomials?  General Math  6  
GPA of Upper Division & Lower Division courses and M.S degree  Academic Guidance  1  
Division with variables (I think I'd call this double division)  Precalculus Mathematics Homework  5 