Dynamic Geometry: P52

Geometry Level pending

The diagram shows a black circle with radius 1 1 . A blue horizontale chord moves vertically, dividing the circle into two circular segments. We inscribe a purple equilateral triangle in the upward circular segment, we also draw its incircle in green. A cyan circle is drawn, it is internally tangent to the black circle and tangent to the triangle and to the blue chord. When the ratio of the cyan circle's radius to the radius of the green incircle is equal to 6 7 \dfrac{6}{7} , the sum their areas can be expressed as p q π \dfrac{p}{q}\pi , where p p and q q are positive integers. Find p + q p+q .


The answer is 661.

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1 solution

Label the centers of the large and cyan circles O O and P P respectively, the triangle A B C ABC , its median A M AM , and P L C = P N O = 9 0 \angle PLC = \angle PNO = 90^\circ . Let the radius of the green circle be r r , then the radius of the cyan circle is 6 7 r \frac 67r .

Since the center of the green circle is centroid of the equilateral A B C \triangle ABC , the median A M = 3 r AM = 3r . Let O M = a OM = a . Then 3 r + a = 1 a = 1 3 r 3r+a = 1 \implies a = 1-3r .

Consider the right O P N \triangle OPN , by Pythagorean theorem ,

O N 2 + P N 2 = O P 2 M L 2 + ( P L + L N ) 2 = O P 2 ( M C + C L ) 2 + ( P L + L N ) 2 = O P 2 ( 3 r + 6 7 3 r ) 2 + ( 6 7 r + a ) 2 = ( 1 6 7 r ) 2 Note that a = 1 3 r ( 21 + 6 7 3 r ) + ( 1 15 7 r ) 2 = ( 1 6 7 r ) 2 243 49 r 2 + 1 30 7 r + 225 49 r 2 = 1 12 7 r + 36 49 r 2 432 49 = 18 7 r Since r > 0 r = 7 24 \begin{aligned} ON^2+PN^2 & = OP^2 \\ ML^2 + (PL+LN)^2 & = OP^2 \\ (MC+CL)^2 + (PL+LN)^2 & = OP^2 \\ \left(\sqrt 3r + \frac 6{7\sqrt 3}r\right)^2 + \left(\frac 67r + \blue a \right)^2 & = \left(1 - \frac 67 r\right)^2 & \small \blue{\text{Note that }a = 1-3r} \\ \left(\frac {21+6}{7\sqrt 3}r\right) + \left(1 - \frac {15}7r \right)^2 & = \left(1 - \frac 67 r\right)^2 \\ \frac {243}{49} r^2 + 1 - \frac {30}7r + \frac {225}{49}r^2 & = 1 - \frac {12}7 r + \frac {36}{49}r^2 \\ \frac{432}{49} & = \frac {18}7 r & \small \blue{\text{Since }r > 0} \\ \implies r & = \frac 7{24} \end{aligned}

Therefore the sum of areas of the two circles is π r 2 + π ( 6 7 r ) 2 = 49 + 36 49 × 49 576 π = 85 576 π \pi r^2 + \pi \left(\dfrac 67r\right)^2 = \dfrac {49+36}{49} \times \dfrac {49}{576} \pi = \dfrac {85}{576}\pi . The required answer p + q = 85 + 576 = 661 p+q = 85+576 = \boxed{661} .

Thank you for posting !

Valentin Duringer - 3 months, 1 week ago

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You are welcome

Chew-Seong Cheong - 3 months, 1 week ago

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