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Equation not balancing

Posted on 2014-01-07
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Last Modified: 2014-01-07
I have the following expression: (3a^-2 * b)/(-3a^2).  To my understanding this expression can be simplified to (-a^-4 * b) because:
3/3 = 1
a^-2/a^2 = a^-4
b remains the same
When I substitute actual values for the variables the equation doesn't balance.
a=2, b=4
-(3a^-2 * b)/(-3a^2) = -a^-4 * b
-(3(0.25)(4))/(-3(4)) = (0.0625)(4)
-3/-12 = 0.25
-0.25 = 0.25
Why is there a polarity difference?  What am I not understanding?
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Question by:NevSoFly
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7 Comments
 
LVL 81

Expert Comment

by:byundt
ID: 39762687
Double-check step 1 in your simplification. You should have a minus sign on the bottom.

(3)/(-3) = -1
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LVL 81

Expert Comment

by:byundt
ID: 39762701
And when you plugged values into the original equation, you put a minus sign in two places on the left side, where the original expression only had 1.
0
 

Author Comment

by:NevSoFly
ID: 39762730
I'm sorry both of those were typos.  The steps should be as below.  The equation still doesn't balance.

•(3a^-2 * b)/(-3a^2) = -a^-4 * b


•(3(0.25)(4))/(-3(4)) = (0.0625)(4)


•3/-12 = 0.25


•-0.25 = 0.25
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LVL 17

Accepted Solution

by:
Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 2000 total points
ID: 39762841
You have a minus sign on the right hand side at -a^-4*b but you lose it when you get to (0.0625)(4).
The minus value should still be there.
The negative you might think you are losing is the index, not the value. The negative index just means 1/(value with positive index)
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LVL 81

Expert Comment

by:byundt
ID: 39762868
[3a^-2*b]/(-3a^2) = -a^-4*b

Plug in a = 2 and b = 4
[3*4/(2^2)] / (-3 * 2^2) = -4/(2^4)

[3*4/4] / (-3*4) = -4/16

[3]/(-12) = -.25

-.25 = -.25
0
 
LVL 17
ID: 39762887
An example in case it helps:
You have -3a^2 which comes out to -3*2^2 = -12, but on your right hand side you have
-a^-4 which could be written as -1a^-4 = -1*0.0625 = -0.0625.
Hope this helps.
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Author Closing Comment

by:NevSoFly
ID: 39762899
That was right in front of my face and I didn't see it,  Thanks.
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