Last visit was: 18 Nov 2025, 14:03 It is currently 18 Nov 2025, 14:03
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 18 Nov 2025
Posts: 105,355
Own Kudos:
778,061
 [5]
Given Kudos: 99,964
Products:
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 105,355
Kudos: 778,061
 [5]
1
Kudos
Add Kudos
4
Bookmarks
Bookmark this Post
Most Helpful Reply
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 18 Nov 2025
Posts: 105,355
Own Kudos:
778,061
 [3]
Given Kudos: 99,964
Products:
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 105,355
Kudos: 778,061
 [3]
1
Kudos
Add Kudos
2
Bookmarks
Bookmark this Post
General Discussion
User avatar
Kinshook
User avatar
Major Poster
Joined: 03 Jun 2019
Last visit: 18 Nov 2025
Posts: 5,793
Own Kudos:
5,508
 [2]
Given Kudos: 161
Location: India
GMAT 1: 690 Q50 V34
WE:Engineering (Transportation)
Products:
GMAT 1: 690 Q50 V34
Posts: 5,793
Kudos: 5,508
 [2]
1
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
User avatar
Lizaza
Joined: 16 Jan 2021
Last visit: 17 Nov 2025
Posts: 165
Own Kudos:
219
 [1]
Given Kudos: 5
GMAT 1: 710 Q47 V40
GMAT 1: 710 Q47 V40
Posts: 165
Kudos: 219
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
The task at hand is solved pretty easily using the Pythagorean theorem and using the data provided under Condition 1.

I attach the image of the geometrical structure that's helpful - but basically, we should draw three radiuses from the centre: two to the tangent points of the circle and one leading straight to A.
As it's quite easy to notice from the drawing, if we know the coordinates of A, we also know that BC equals to radius minus A's height (so, BK - 8, or R - 8).
Same applies to the horizontal radius: CH is of course parallel and equal to the radius, so AC = HC - A's X-coordinate, or AC = HC - 4 = R - 4.

Now let's look at the triangle ABC:
\(AB^2 = AC^2 + BC^2\\
R^2 = (R-4)^2 + (R-8)^2\\
R^2 = R^2 + 16 -8R + R^2 + 64 - 16R\\
R^2 = 2R^2 - 24R + 80,\)
or we get a simple quadratic equation:
\(R^2 - 24R + 80 = 0\)

It's easy to distinguish, using the Vieta's formulas, that the roots are 4 and 20. So, we know that there are two options for the radius.

And now it looks tempting to use the Condition 2, which tells us to choose the option above 15, which is 20.
However, this condition is excessive, because if the radius equals to 4, than the point A would be placed in a completely different part of the circle - to be precise, on the top on the diameter drawn from point K in my scheme.
This is not possible for two reasons:
    (1) we were given the drawing where the placement of A is quite obvious; and
    (2) we actually obtained the equation using A the way it's drawn - so if it's in any other place, the whole geometrical solution wouldn't have made any sense.

Therefore, we don't need any help to understand that the only possible root for the radius is 20. So, Condition 1 is enough - answer A.
Attachments

File comment: The updates drawing with solution.
task scheme.jpg
task scheme.jpg [ 74.37 KiB | Viewed 1159 times ]

User avatar
EthanTheTutor
Joined: 08 Jun 2022
Last visit: 14 Nov 2025
Posts: 43
Own Kudos:
Given Kudos: 13
Location: United States
GMAT 1: 780 Q51 V48
GMAT 1: 780 Q51 V48
Posts: 43
Kudos: 72
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
Bunuel
12 Days of Christmas GMAT Competition with Lots of Fun


In the given figure both the co-ordinate axes are tangent to the drawn circle. If A is a point on the circumference of the circle as shown, then what is the radius of the circle?

(1) The co-ordinate of A is (4, 8).
(2) The radius is greater than 15.

 


This question was provided by GMATWhiz
for the 12 Days of Christmas Competition.

Win $30,000 in prizes: Courses, Tests & more

 



Attachment:
The attachment 2022-12-12_18-00-51.png is no longer available

GMATWhiz Official Explanation:

Step 1: Analyse Question Stem:
    • We are given the figure.

      o The co-ordinate axes are tangent to the drawn circle.
      o If A is a point on the circumference of the circle as shown.
    • We need to find the radius of the circle.
      o Let the radius = r.
    • Now if the center is O, then the coordinate of O should be (r, r) as we can see in the following figure.


Step 2: Analyse Statements Independently (And eliminate options) – AD/BCE

Statement 1: The co-ordinate of A is (4, 8).
    • The co-ordinate of A is (4,8)
    • We can say that OA is the radius and thus it should be = r.
      o Thus, distance between O and A = \(\sqrt{(r-8)^2+(r-4)^2} = r\)
      \(\sqrt{(r-8)^2+(r-4)^2} = r\)
      Or, \((r-8)^2+(r-4)^2 = r^2\) (Squaring each side)
      Or, \(r^2-16r+64+ r^2-8r+16 = r^2\)
      Or, \(r^2-24r+80 = 0\)
      Or, \((r – 20) (r – 4) = 0\)
      Or, r = 20 or 4.
    • One can infer that since we have two possibilities so this statement is insufficient.
    • However, we know the point A has a co-ordinate of (4, 8).
      o And as per the figure r must be greater than both 4 and 8.
      o Thus, only possible value is 20.
    This statement is sufficient and we eliminate options B, C, and E.

Statement 2: The radius is greater than 15.
    • We can see that there are infinite possibilities for radius from this info.
This statement is insufficient and thus we eliminate option D.

The answer is option A.

Attachment:
The attachment 2022-12-20_16-11-02.png is no longer available

This solution is incorrect. Statement 1 is not sufficient, as shown in the picture I've attached.
Attachments

Screenshot 2023-01-05 083702.png
Screenshot 2023-01-05 083702.png [ 138.5 KiB | Viewed 974 times ]

Moderators:
Math Expert
105355 posts
496 posts