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PS: Electric circuit (OG)

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PS: Electric circuit (OG) [#permalink] New post 12 Apr 2006, 05:18
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Hi- I understand the algebra in the problem below, but I find the wording very tricky. Do others understand what is being asked here?

In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?
A. xy
B. x + y
C. 1/(x+y)
D. xy/(x+y)
E. (x+y)/xy
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 [#permalink] New post 12 Apr 2006, 05:25
I would simply concentrate on the following extract.... I think it is enough to answer the question.

....the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?
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 [#permalink] New post 12 Apr 2006, 05:25
D.


The wordings look fine.
The Qsays that r is combined resistance of x y, and the relation of r, x, y is such that..... so find R

1/R =1/X +1/Y

1/R = (X+Y)/XY

R= XY/(X+Y)
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 [#permalink] New post 12 Apr 2006, 05:30
SimaQ wrote:
I would simply concentrate on the following extract.... I think it is enough to answer the question.

....the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?


right. (SunShine - you have the correct answer too)
But what about "r is the combined resistance of these two resistors"
Wouldn't that say r = x+y? I understand that the solution wouldn't be that simple, but I couldn't grasp how r could be the combination of the resistances, while the reciprocal of r is equal to the sum of the reciprocals of the resistances.
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 [#permalink] New post 12 Apr 2006, 05:44
This a simple physics formula?

Where did u get this question from? Looks like ETS started asking simple physics problem in GMAT!

I saw another problem on kinematics somewhere using the formula:

S = ut + 1/2 * a (t^2)
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 [#permalink] New post 12 Apr 2006, 08:20
sm176811 wrote:
This a simple physics formula?

Where did u get this question from? Looks like ETS started asking simple physics problem in GMAT!

I saw another problem on kinematics somewhere using the formula:

S = ut + 1/2 * a (t^2)



This is from the OG 11. Q230 or so i beleive.
I'm not sure this is actual physics. Even though the questing is talking about circuits, it is asking you to determine their relationship algebraically. The question could have used any domain - it is the relationship that is important
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 [#permalink] New post 12 Apr 2006, 08:28
No, the relationship is not important! But helps if u have the bacground.

There was a problem abt a ball being thrown up. U had to make some assumption abt heights, with wud have been obvious if u had done physics!

Let me see whether i can find that problem.
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 [#permalink] New post 12 Apr 2006, 09:56
kook44 wrote:
SimaQ wrote:
I would simply concentrate on the following extract.... I think it is enough to answer the question.

....the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?


right. (SunShine - you have the correct answer too)
But what about "r is the combined resistance of these two resistors"
Wouldn't that say r = x+y? I understand that the solution wouldn't be that simple, but I couldn't grasp how r could be the combination of the resistances, while the reciprocal of r is equal to the sum of the reciprocals of the resistances.


It does not matter. You should concentrate on the sentence that says that reciprocal of r is equal to the sum of reciprocals of x and y. It is a stated fact so we can construct a simple equation and solve it....
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 [#permalink] New post 14 Apr 2006, 11:07
if r is the combined resistance of these two resistors
Here it does not mean X+Y=R, this is an electrical term saying the total resistance of the circuit is, where as its total is obtained by the reprocal statement.
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  [#permalink] 14 Apr 2006, 11:07
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