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Math Expert V
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C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Question Stats: 26% (02:00) correct 74% (01:29) wrong based on 150 sessions

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C is a circle with center D and radius 2. E is a circle with center F and radius R. Are there any points that are on both E and C?

(1) The distance from D to F is 1 + R.
(2) R = 3.

Source: Barron's GMAT

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Re: C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Bunuel wrote:
C is a circle with center D and radius 2. E is a circle with center F and radius R. Are there any points that are on both E and C?

(1) The distance from D to F is 1 + R.
(2) R = 3.

Source: Barron's GMAT

Statement 2 is clearly not sufficient.

Per statement 1, distance DF = 1+R , now if R=3, then you do get some common points but if R=0.3, then the smaller circle will lie completely inside the bigger circle , giving you 0 common points. Thus this statement is not sufficient.

Combining, you get R=3, DF=4 and the only case is for the 2 circles to intersect at 2 points, giving you a "yes" for common points between the circle.

Thus C is the correct answer.
Current Student Joined: 09 Nov 2014
Posts: 52
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GMAT 1: 760 Q51 V41 C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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So we're given the dimensions of circle C and then we're told circle F has the radius R.

1 tells us that the distance between the two circles is 1+R. If the two circles were separated by more than their radii combined, it means that they don't intersect. However, 1+R is less than the radius of circle C and the radius of Circle F combined (2+R). Therefore, circle C and circle F intersect at 2 points (if 1+R=radius of circle C and circle F, it would be tangent).

SUFFICIENT

2 tells us the value of R but gives us no indication about the location of circle F in relation to circle C.

INSUFFICIENT

Therefore the answer is A

Edit: don't mind me, my answer is wrong

Originally posted by jmikery on 25 Oct 2015, 07:19.
Last edited by jmikery on 25 Oct 2015, 08:02, edited 1 time in total.
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C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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jmikery wrote:
So we're given the dimensions of circle C and then we're told circle F has the radius R.

1 tells us that the distance between the two circles is 1+R. If the two circles were separated by more than their radii combined, it means that they don't intersect. However, 1+R is less than the radius of circle C and the radius of Circle F combined (2+R). Therefore, circle C and circle F intersect at 2 points (if 1+R=radius of circle C and circle F, it would be tangent).

SUFFICIENT

2 tells us the value of R but gives us no indication about the location of circle F in relation to circle C.

INSUFFICIENT

Therefore the answer is A

IMO, you can not say statement 1 is sufficient without knowing the exact value of R. Case in point R=3 or R=0.3

You will get 2 different answers for the above 2 values of R.
Current Student Joined: 09 Nov 2014
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GMAT 1: 760 Q51 V41 Re: C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Engr2012 wrote:
jmikery wrote:
So we're given the dimensions of circle C and then we're told circle F has the radius R.

1 tells us that the distance between the two circles is 1+R. If the two circles were separated by more than their radii combined, it means that they don't intersect. However, 1+R is less than the radius of circle C and the radius of Circle F combined (2+R). Therefore, circle C and circle F intersect at 2 points (if 1+R=radius of circle C and circle F, it would be tangent).

SUFFICIENT

2 tells us the value of R but gives us no indication about the location of circle F in relation to circle C.

INSUFFICIENT

Therefore the answer is A

IMO, you can not say statement 1 is sufficient without knowing the exact value of R. Case in point R=3 or R=0.3

You will get 2 different answers for the above 2 values of R.

Here's my POV on it:

No matter what R is, the circles won't touch if the distance between the centers of the two circles is greater than combined radii, R+2, in this case (R being the radius of circle F).

However, statement 1 says that the distance between the centers is R+1, which is shorter than the combined distance of the radii. Therefore, no matter what R is, R+1 is always less than the distance between the two radii and therefore: the circles have to intersect at some point.

Given the two cases: If R=3, the circles are 3+1=4 units apart. The radii are a combined 5 units, therefore the circles intersect.

If R=0.3, the circles are 0.3+1=1.3 units apart. The radii are a combined 2.3 units, therefore the distance is still smaller than the combined length of the radii and thus the circles intersect.

Whatever the value of R, the combined length of the radii will be (R+2)-(R+1)=1 unit larger than the distance between the circles.
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C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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jmikery wrote:

Here's my POV on it:

No matter what R is, the circles won't touch if the distance between the centers of the two circles is greater than combined radii, R+2, in this case (R being the radius of circle F).

However, statement 1 says that the distance between the centers is R+1, which is shorter than the combined distance of the radii. Therefore, no matter what R is, R+1 is always less than the distance between the two radii and therefore: the circles have to intersect at some point.

Given the two cases: If R=3, the circles are 3+1=4 units apart. The radii are a combined 5 units, therefore the circles intersect.

If R=0.3, the circles are 0.3+1=1.3 units apart. The radii are a combined 2.3 units, therefore the distance is still smaller than the combined length of the radii and thus the circles intersect.

Whatever the value of R, the combined length of the radii will be (R+2)-(R+1)=1 unit larger than the distance between the circles.

For R=0.3, the radii will add up to 2.3 but as the centers will only be 1.3 units apart, the smaller circle will completely lie and not tangentially inside the bigger circle. Hence 0 common points.
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GMAT 1: 760 Q51 V41 C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Engr2012 wrote:
jmikery wrote:

Here's my POV on it:

No matter what R is, the circles won't touch if the distance between the centers of the two circles is greater than combined radii, R+2, in this case (R being the radius of circle F).

However, statement 1 says that the distance between the centers is R+1, which is shorter than the combined distance of the radii. Therefore, no matter what R is, R+1 is always less than the distance between the two radii and therefore: the circles have to intersect at some point.

Given the two cases: If R=3, the circles are 3+1=4 units apart. The radii are a combined 5 units, therefore the circles intersect.

If R=0.3, the circles are 0.3+1=1.3 units apart. The radii are a combined 2.3 units, therefore the distance is still smaller than the combined length of the radii and thus the circles intersect.

Whatever the value of R, the combined length of the radii will be (R+2)-(R+1)=1 unit larger than the distance between the circles.

For R=0.3, the radii will add up to 2.3 but as the centers will only be 1.3 units apart, the smaller circle will completely lie and not tangentially inside the bigger circle. Hence 0 common points.

Ohhh okay, I get it now, I was wrongly attributing the extra +1 to R as part of the radius. Am I correct in saying that we need to know if R>0.5 to confirm if the circles touch? If R is > or = to 0.5, then the center F is at least 1.5 units away from center D and the opposite end of circle E will touch some point of circle C given that it's at least 1.5+0.5=2 units from center D
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C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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jmikery wrote:
Engr2012 wrote:
jmikery wrote:

Here's my POV on it:

No matter what R is, the circles won't touch if the distance between the centers of the two circles is greater than combined radii, R+2, in this case (R being the radius of circle F).

However, statement 1 says that the distance between the centers is R+1, which is shorter than the combined distance of the radii. Therefore, no matter what R is, R+1 is always less than the distance between the two radii and therefore: the circles have to intersect at some point.

Given the two cases: If R=3, the circles are 3+1=4 units apart. The radii are a combined 5 units, therefore the circles intersect.

If R=0.3, the circles are 0.3+1=1.3 units apart. The radii are a combined 2.3 units, therefore the distance is still smaller than the combined length of the radii and thus the circles intersect.

Whatever the value of R, the combined length of the radii will be (R+2)-(R+1)=1 unit larger than the distance between the circles.

For R=0.3, the radii will add up to 2.3 but as the centers will only be 1.3 units apart, the smaller circle will completely lie and not tangentially inside the bigger circle. Hence 0 common points.

Ohhh okay, I get it now, I was wrongly attributing the extra +1 to R as part of the radius. Am I correct in saying that we need to know if R>0.5 to confirm if the circles touch? If R is > or = to 0.5, then the center F is at least 1.5 units away from center D and the opposite end of circle E will touch some point of circle C

Yes, exactly. You need R$$\geq$$ 0.5 (and not just R>0.5 as R=0.5 will also give you 1 common points, the point of tangency) is what you need to be absolutely sure. No matter what value of R you assume > 0.5, it will always lead to 2 intersecting circles as the distance between the centers will be 1+R. Additionally, although statement 2 wasnt sufficient on its own, it should provide you the hint that something is going on with this particular value of R=3.

In GMAT DS questions, both statements should lead to the same unique answer. You can not have 2 contradictory unique answers.

Hope this helps.
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GMAT 1: 760 Q51 V41 Re: C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Engr2012 wrote:
jmikery wrote:
Engr2012 wrote:
For R=0.3, the radii will add up to 2.3 but as the centers will only be 1.3 units apart, the smaller circle will completely lie and not tangentially inside the bigger circle. Hence 0 common points.

Ohhh okay, I get it now, I was wrongly attributing the extra +1 to R as part of the radius. Am I correct in saying that we need to know if R>0.5 to confirm if the circles touch? If R is > or = to 0.5, then the center F is at least 1.5 units away from center D and the opposite end of circle E will touch some point of circle C

Yes, exactly. R>0.5 is what you need to be absolutely sure. No matter what value of R you assume > 0.5, it will always lead to 2 intersecting circles as the distance between the centers will be 1+R. Additionally, although statement 2 wasnt sufficient on its own, it should provide you the hint that something is going on with this particular value of R=3.

In GMAT DS questions, both statements should lead to the same unique answer. You can not have 2 contradictory unique answers.

Hope this helps.

Awesome, thanks for the explanation!
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Re: C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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Bunuel wrote:
C is a circle with center D and radius 2. E is a circle with center F and radius R. Are there any points that are on both E and C?

(1) The distance from D to F is 1 + R.
(2) R = 3.

Source: Barron's GMAT

I will go with option C.

statement 1 : if D = 2R < 1 then the answer will be NO. And if D = 2R >=1 then the answer is Yes. There is no definite answer , and hence not sufficient.
Statement 2 : This is clearly not sufficient. These two circle may intersect or may be independent completely.

combination: from statement one we have to check whether 2R< 1 ? And from statement 2 we have R = 3 i.e 2R = 6 and Is 6 < 1. The answer is no. Hence the circle will intersect. Correct option C.
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Re: C is a circle with center D and radius 2. E is a circle with center F  [#permalink]

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jmikery wrote:
Engr2012 wrote:
jmikery wrote:
So we're given the dimensions of circle C and then we're told circle F has the radius R.

1 tells us that the distance between the two circles is 1+R. If the two circles were separated by more than their radii combined, it means that they don't intersect. However, 1+R is less than the radius of circle C and the radius of Circle F combined (2+R). Therefore, circle C and circle F intersect at 2 points (if 1+R=radius of circle C and circle F, it would be tangent).

SUFFICIENT

2 tells us the value of R but gives us no indication about the location of circle F in relation to circle C.

INSUFFICIENT

Therefore the answer is A

IMO, you can not say statement 1 is sufficient without knowing the exact value of R. Case in point R=3 or R=0.3

You will get 2 different answers for the above 2 values of R.

Here's my POV on it:

No matter what R is, the circles won't touch if the distance between the centers of the two circles is greater than combined radii, R+2, in this case (R being the radius of circle F).

However, statement 1 says that the distance between the centers is R+1, which is shorter than the combined distance of the radii. Therefore, no matter what R is, R+1 is always less than the distance between the two radii and therefore: the circles have to intersect at some point.

Given the two cases: If R=3, the circles are 3+1=4 units apart. The radii are a combined 5 units, therefore the circles intersect.

If R=0.3, the circles are 0.3+1=1.3 units apart. The radii are a combined 2.3 units, therefore the distance is still smaller than the combined length of the radii and thus the circles intersect.

Whatever the value of R, the combined length of the radii will be (R+2)-(R+1)=1 unit larger than the distance between the circles.

I guess this solution works for circles external to each other...................... Re: C is a circle with center D and radius 2. E is a circle with center F   [#permalink] 02 Aug 2019, 03:54
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