As the figure above shows, two circles lie on a line and tangent to ea

Question

As the figure below shows, two circles lie on a line and tangent to each other. If the radius of two circles are 2r and r, respectively, in terms of r, AB=?

(A)  \frac{5r}{2} 
(B)  \frac{8r}{3} 
(C)  2*\sqrt{2}*r 
(D)  3r 
(E)  2*\sqrt{3}*r

Solutions without formula based approach would be appreciated.

Wonderwoman31 · Answer

Gladiator59 ,  generis .

Hi,

Could you please help with this question. With my approach, I am not able to come up with any of the answer choices. My answer is (3^ 1/2 + 1)*r

m1033512 · Answer

IMO C

there is a direct formula to calculate the length of common tangent
 ^1/2

^1/2

solving this we get option C

stne · Answer

Hi  MBA20 ,
Can you clarify, what is A and B? Is A the area of circle A and B the area of circle B? Or is AB the distance between points A and B in the circle.

nick1816 · Answer

PR= 2r+r=3r
PS= 2r-r=r
 AB^2= SR^2= PR^2 - PS^2= (3r)^2 - r^2 = 8r^2 
 AB= 2*2^{1/2} *r

generis · Answer

tangcircles2.jpg Wonderwoman31 , let me know if the approach below does not make sense. stne , it's hard to tell exactly, but the question tests tangent lines, so A and B are the points of tangency on the exterior tangent line. MBA20 , as requested, no formulas. (I don't blame you. I don't use distance formulas when I can avoid doing so. This distance formula is very handy. It's in the footnote*, but as you can see, you don't need it.) Essentially, connect the centers; make a quadrilateral using perpendicular radii; divide that quadrilateral into a rectangle and a right triangle; and solve for the unknown side length of the right triangle (MQ). That length = AB. Properties of two circles that are tangent to each other • The circles touch each other at exactly one point and lie on the same line PQ • The circles have three common tangent lines: 1) one internal "common" tangent line and 2) two external tangent lines (see little drawing in blue, lower left of diagram). For these two circles, the bottom line (LINE AB) is an exterior tangent line. • The radius from the center of each circle is perpendicular to the tangent line, on which lies the point of tangency. • Derive parallel and perpendicular lines in the diagram from the statement directly above Circles P and Q are both tangent to LINE AB at points A and B, respectively. Make a quadrilateral, then make a right triangle. Divide the quadrilateral into a rectangle and a right triangle. 1) Connect P and Q 2) Drop a line from P to A and from Q to B -- Those lines are perpendicular to LINE AB. PA and QB are radii of each circle and therefore perpendicular to LINE AB at points A and B -- Those radii are parallel: each radius PA and QB is perpendicular to LINE AB. Two lines that are perpendicular to the same line are parallel to each other. 3) Now we have a quadrilateral APQB 4) Draw a line from the center of Q to point M; that line is parallel to the bottom line (external tangent line AB) 5) We have rectangle AMQB and right ∆ MPQ 6) The base of the right triangle, MQ , is the length of AB (rectangle has sides of equal length) 7) Let AB = MQ = x Right ∆ MPQ • Length of side PQ , hypotenuse = (2r + r) = 3r • Length of side MP = (2r - r) = r -- Subtract the rectangle side length from the radius to get MP, that is -- (PA - MA)= MP PA = 2r MA = QB = r • use Pythagorean theorem to find length of third side, x Pythagorean theorem MP^2 + x^2 = PQ^2 r^2 + x^2 = (3r)^2 r^2 + x^2 = 9r^2 x^2 = 8r^2 \sqrt{x^2} =\sqrt{8r^2} x = 2r\sqrt{2} Rewrite: x = MQ = AB = 2\sqrt{2}*r ANSWER C Hope that helps. *From what I just did, you can derive a simpler version of the formula that m1033512 used +1. a = longer radius (in this problem, a = 2r ) b = shorter radius (in this problem, b = r ) The distance between base points of two circles tangent to each other is \sqrt{(a+b)^2 - (a-b)^2}=2\sqrt{ab} EDIT : nick1816 , +1.

MBA20 · Answer

A is a point where the bigger circle intersects the line and B is a point where the smaller circle intersects the line.

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

As the figure above shows, two circles lie on a line and tangent to ea

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

GMAT Problem Solving (PS) Questions

As the figure above shows, two circles lie on a line and tangent to ea

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia