Dynamic Square Geometry

Geometry Level 3

The diagram shows two tangential squares whose vertex exists at the endpoint of the diameter of semicircle of unit radius length. As their vertices on the circumference move counterclockwise, the cyan square decreases in size, whereas the purple square increases in size.

Find the area sum of the cyan and the purple squares at the moment when the center of the semicircle lies on the perimeter of the purple square.

6 5 \dfrac{6}{5} 7 5 \dfrac{7}{5} 8 5 \dfrac{8}{5} 6 5 \dfrac{6}{\sqrt{5}} 7 5 \dfrac{7}{\sqrt{5}} 8 5 \dfrac{8}{\sqrt{5}} None of the given choices.

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2 solutions

Sathvik Acharya
Mar 25, 2021

Let the side length of the purple and cyan square be x x and y y , respectively. Since, C M = B E = x , B E O = C M O = 9 0 CM=BE=x,\; \angle BEO=\angle CMO=90^{\circ} and O C = O B = 1 OC=OB=1 , by RHS congruence criterion , C M O B E O O E = O M = x 2 \triangle CMO\cong \triangle BEO \\ \implies OE=OM=\frac{x}{2}\;\;\;\;\;\;\;\;\;\;\;\; Using the Pythagorean Theorem , in C M O \triangle CMO , O M 2 + C M 2 = O C 2 x 2 4 + x 2 = 1 x = 2 5 \begin{aligned} OM^2+CM^2&=OC^2 \\ \frac{x^2}{4}+x^2&=1 \\ \therefore \; x&=\frac{2}{\sqrt{5}} \end{aligned} Note, A C B = M C B = 9 0 \angle ACB=\angle MCB=90^{\circ} since angle in a semicircle is right angle and M C B E MCBE is a square. Hence, the points A , M , C A,M,C are collinear and M M is the foot of the perpendicular from the center O O to the chord A C AC . Therefore, M A = M C y 2 = x y = 2 5 \begin{aligned} MA&=MC \\ y\sqrt{2}&=x\\ \therefore \; y&=\sqrt{\frac{2}{5}} \end{aligned} The sum of the area of the two squares is, x 2 + y 2 = 4 5 + 2 5 = 6 5 x^2+y^2=\dfrac{4}{5}+\dfrac{2}{5}=\boxed{\dfrac{6}{5}}

Agent T
Mar 29, 2021

I took a screenshot when the purple square's side intersected the centre And Then just by looking at the pic I assumed the ans to be 6 5 \boxed{\frac{6}{5}} which was correct:D

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