Three Circles touching each other

Geometry Level 3

The three r e d \red{red} circles are congruent with radius R c m R\space cm . Line P Q \overline{PQ} is tangent to the lower circle at point T T such that it is parallel to the line joining the centers of the above two circles. Line R S \overline{RS} is a direct common tangent between the two circle shown in the figure.

If area of quadrilateral P Q R S PQRS is 10 c m 2 10 \space cm^2 . Then find integer nearest to R 2 R^2


Inspiration


The answer is 14.

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1 solution

David Vreken
Feb 7, 2021

Place the circles on a coordinate plane so that the center between the three circles is at (0, 0). Then the left circle has an equation of ( x + R ) 2 + ( y 3 3 R ) 2 = R 2 (x + R)^2 + (y - \frac{\sqrt{3}}{3}R)^2 = R^2 , the right circle ( x R ) 2 + ( y 3 3 R ) 2 = R 2 (x - R)^2 + (y - \frac{\sqrt{3}}{3}R)^2 = R^2 , and the bottom circle x 2 + ( y + 2 3 3 R ) 2 = R 2 x^2 + (y + \frac{2\sqrt{3}}{3}R)^2 = R^2 .

The y y -coordinate of T T can be found by solving 0 2 + ( T y + 2 3 3 R ) 2 = R 2 0^2 + (T_y + \frac{2\sqrt{3}}{3}R)^2 = R^2 , which is T y = ( 1 2 3 3 ) R T_y = (1 - \frac{2\sqrt{3}}{3})R .

The x x -coordinate of Q Q can be found by solving ( Q x R ) 2 + ( T y 3 3 R ) 2 = R 2 (Q_x - R)^2 + (T_y - \frac{\sqrt{3}}{3}R)^2 = R^2 , which is Q x = ( 2 3 3 + 1 ) R Q_x = (\sqrt{2\sqrt{3}-3}+1)R

The x x -coordinate of R R is R x = R R_x = R .

The y y -coordinate of R R can be found by solving R x 2 + ( R y + 2 3 3 R ) 2 = R 2 R_x^2 + (R_y + \frac{2\sqrt{3}}{3}R)^2 = R^2 , which is R y = ( 3 3 1 ) R R_y = (\frac{\sqrt{3}}{3} - 1)R .

The height of the trapezoid is then h = T y R y = ( 2 3 ) R h = T_y - R_y = (2 - \sqrt{3})R .

The median of the trapezoid is then m = 1 2 ( 2 Q x + 2 R x ) = ( 2 3 3 + 2 ) R m = \frac{1}{2}(2Q_x + 2R_x) =(\sqrt{2\sqrt{3}-3}+2)R

The area of the trapezoid is A = ( 2 3 ) ( 2 3 3 + 2 ) R 2 = 10 A = (2 - \sqrt{3})(\sqrt{2\sqrt{3}-3}+2)R^2 = 10 , which solves to R 2 13.919 14 R^2 \approx 13.919 \approx \boxed{14} .

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