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Old 20th March 2007, 12:05 AM   #1
phn is offline phn  Sweden
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Default Math by armbar

I have no option but to concede defeat.

Some math wiz here that can explain what "A//B" is? It is supposed to means A divided by B in parallel. The real question for me is, how do I type it in my graphing calculator?
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Old 20th March 2007, 12:13 AM   #2
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Default Re: Math by armbar

Quote:
Originally posted by phn
... A divided by B in parallel...
I don't even understand what that means. Could you show an example?
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Old 20th March 2007, 12:23 AM   #3
phn is offline phn  Sweden
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Quote:
Confirm the total RL value for chosen RLdc and RLac in parallel.
RL total = 47k // 100k = 32k.
From a tube load lines tutorial.

Thinking about it, I'm not quite sure where I got what I wrote in my original post. I blame tired head and eyes after hours of reading and re-reading.
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Old 20th March 2007, 10:36 AM   #4
phn is offline phn  Sweden
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Solved it.

X=1/((1/A)+(1/B))

Too tired to think last night, I guess. I solved it by trying to think of some similar equation I might have done in the past. I don't have a clue about what it means. But, as said, I only have to know how to type it in my calculator.

phn by painful heel lock.
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Old 20th March 2007, 04:08 PM   #5
phn is offline phn  Sweden
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Doh!

Not long after I left home did it hit me. It calculates paralleled resistors. I knew that. You use it to set the load for MC carts.
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Old 20th March 2007, 06:16 PM   #6
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Quote:
Originally posted by phn
Solved it.

X=1/((1/A)+(1/B))


The formula full of reciprocals of reciprocals is good if you have three or more items connected in parallel. For just two it is usually easier to remember and calculate A*B/(A+B).

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Old 20th March 2007, 10:12 PM   #7
phn is offline phn  Sweden
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The engineer in you speaking, eh?

And here I was all happy having solved it on my own. Not bad, I think, considering I have forgot, if not everything, 95 pct of what I learned in high school. I think and hope this is HS level. But then, I probably have forgotten 95 pct of everything I learned in school. Thinking about it, I don't know what I learned in school other than being quite and feeling shame if you didn't. Man, what a hellhole it was.
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Old 21st March 2007, 02:18 AM   #8
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If phn's calculator has a reciprocal function key (say "1/X" or x^-1); then the reciprocals of reciprocals is less keystrokes than the reciprocal of the LCD (least common denominator, A*B) form for 2 resistors even if A and B are only 1 digit each. You would enter: A, rec key, +, B, rec key, equal key, rec key

Also the reciprocal of the reciprocal form allows you to do a problem like: I need two resistors in parallel to make 10K and I have a 22K resistor, what do I need?

=> 1/((1/10K)-(1/22K)), without breaking a sweat!

The form A // B => A*B/(A+B), should never be taught; as it has no intuitive connection to conductance => 1/resistance which is at the heart of combining resistors in parallel and understanding how to calculate the answer in general.

Just an observation and opinion, all correct answers are equivalent!
VSR
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Old 21st March 2007, 06:17 AM   #9
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Default Re: Math by armbar

Quote:
Originally posted by phn
can explain what "A//B" is?
I expect that who-ever typed that didn't know where the right key was.... should be A||B -- "A in parallel with B".

1/x = 1/A + 1/B as per your formula...

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Old 21st March 2007, 06:19 AM   #10
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Quote:
Originally posted by sherelec
If phn's calculator has a reciprocal function key (say "1/X" or x^-1); then the reciprocals of reciprocals is less keystrokes than the reciprocal of the LCD (least common denominator, A*B) form for 2 resistors even if A and B are only 1 digit each. You would enter: A, rec key, +, B, rec key, equal key, rec key
Even faster if you use a proper RPN calculator

A, 1/x, B. 1/x + 1/x

dave
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