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Reciprocals

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by TuanNguyen87 » Thu Jun 09, 2011 12:21 am
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

Answer: D

Thanks a lot in advance!!!
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by Frankenstein » Thu Jun 09, 2011 12:23 am
Hi,
(1/r) = (1/x)+(1/y) = (x+y)/xy
So, r = xy(x+y)

Hence, D
Cheers!

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by cans » Thu Jun 09, 2011 12:25 am
1/r = 1/x + 1/y = (x+y)/xy
thus r = xy/(x+y)
IMO D
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by TuanNguyen87 » Thu Jun 09, 2011 12:26 am
Frankenstein wrote:Hi,
(1/r) = (1/x)+(1/y) = (x+y)/xy
So, r = xy(x+y)

Hence, D
Wow you're so smart and quick :shock: But can you explain for me more about reciprocals? I have no idea about this. Thanks so much!!!!
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by TuanNguyen87 » Thu Jun 09, 2011 12:27 am
You guys are really smart :shock: Thanks!!!
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by kcstej » Thu Jun 09, 2011 12:29 am
1/r= 1/x + 1/y
1/r= (x+y)/xy
r= xy/(x+y)

So D
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by Frankenstein » Thu Jun 09, 2011 1:00 am
TuanNguyen87 wrote:
Frankenstein wrote:Hi,
(1/r) = (1/x)+(1/y) = (x+y)/xy
So, r = xy(x+y)

Hence, D
Wow you're so smart and quick :shock: But can you explain for me more about reciprocals? I have no idea about this. Thanks so much!!!!
Hi,
If you have any specific doubt about reciprocals or whatever, you can just PM me.
Cheers!

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by GMATGuruNY » Thu Jun 09, 2011 2:05 am
TuanNguyen87 wrote: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

Answer: D

Thanks a lot in advance!!!
We can plug in values for x and y.

Let x=2, y=3.
Then the reciprocals of x and y are 1/2 and 1/3.
Since the reciprocal of r is equal to the sum of the reciprocals of x and y, we get:
1/r = 1/2 + 1/3
1/r = 3/6 + 2/6
1/r = 5/6
r = 6/5. This is our target.

Now we plug x=2 and y=3 into the answers to see which yields our target of 6/5.

Only answer choice D works:
xy/(x+y) = (2*3)/(2+3) = 6/5.

The correct answer is D.
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by MBA.Aspirant » Thu Jun 09, 2011 5:34 am
TuanNguyen87 wrote: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

Answer: D


Thanks a lot in advance!!!

1/r = 1/x + 1/y

1/r = x+y/xy

r = xy / x+y
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by Buix0065 » Fri Jul 22, 2011 1:54 pm
In this problem it states that r is the combined resistance of x and y.

So why isn't the correct answer X + Y ? Clearly I'm misunderstanding the wording of the question, just wouldn't want to miss something like this on the actual!
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