Inscribing Squares Since 2K

Geometry Level 3

In the above figure, square A B C D ABCD is inscribed in a circle. E F G H EFGH is also a square with points E E and F F on the circle and G G and H H on side A B AB of the bigger square.

Find the ratio of the areas of the bigger square to the smaller square correct to 3 decimal places.


The answer is 25.000.

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1 solution

Construction: Let O O be the centre of the circle. Join O A OA , O E OE and O F OF . Let J J be a point on side E F EF such that O J E F OJ \perp EF . O J OJ intersects side A B AB at I I .

Let the side lengths of the larger and smaller square be A A and a a respectively. Then ( 2 R ) 2 = A 2 + A 2 2 R 2 = A 2 (2R)^2 = A^2 + A^2 \implies 2R^2 = A^2 .

We have O E = O F = R OE = OF = R , so Δ E O F \Delta EOF is isosceles. Hence, O J OJ is also the median to side E F EF . E J = J F = E F 2 = a 2 \implies EJ = JF = \frac{EF}{2} = \frac{a}{2} .

Also, A I O = 9 0 \angle AIO = 90^{\circ} . A I = I B = A 2 \implies AI = IB = \frac{A}{2} and I A O = 4 5 A I = I O I O = A 2 \angle IAO = 45^{\circ} \implies AI = IO \implies IO = \frac{A}{2} .

Now, since Δ E J O \Delta EJO is right angled at J J , applying the Pythagorean Theorem,

E J 2 + J O 2 = O E 2 ( a 2 ) 2 + ( I J + I O ) 2 = R 2 ( a 2 ) 2 + ( a + A 2 ) 2 = R 2 a 2 ( 1 4 ) + a 2 ( 1 + A a 1 2 ) 2 = A 2 2 EJ^2 + JO^2 = OE^2 \implies \left(\frac{a}{2}\right)^2+\left(IJ\ +\ IO\right)^2=R^2 \implies \left(\frac{a}{2}\right)^2+\left(a\ +\ \frac{A}{2}\right)^2=R^2 \implies a^2\left(\frac{1}{4}\right)+a^2\left(1+\ \frac{A}{a}\cdot \frac{1}{2}\right)^2=\frac{A^2}{2}

Dividing throughout by a 2 a^2 , we have:

( 1 4 ) + ( 1 + A a 1 2 ) 2 = ( A a ) 2 1 2 \left(\frac{1}{4}\right)+\left(1+\ \frac{A}{a}\cdot \frac{1}{2}\right)^2=\left(\frac{A}{a}\right)^2\cdot \frac{1}{2}

Let A a = k \frac{A}{a} = k , then the above equation becomes:

1 4 + ( 1 + k 2 ) 2 = k 2 2 \frac{1}{4}+\left(1+\ \frac{k}{2}\right)^2=\frac{k^2}{2}

Solving the above quadratic equation, we have k = 5 k=5 or 1 -1 . We reject 1 -1 as the ratio of the sides cannot be negative. So, k = 5 A a = 5 k = 5 \implies \boxed{\frac{A}{a} = 5} . Hence, ratio of their areas = A 2 a 2 = ( A a ) 2 = 5 2 = 25 = \frac{A^2}{a^2} = \left(\frac{A}{a}\right)^2 = 5^2 = \boxed{25}

Sidenote here: the YouTube channel "Mind Your Decision" recently featured this question in his video. This is the link: (https://www.youtube.com/watch?v=NalDbjj2bL4).

Toby M - 3 years, 10 months ago

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I know that. I still posted my solution to this.

Arkajyoti Banerjee - 3 years, 10 months ago

Nice question and solution.

Hana Wehbi - 3 years, 10 months ago

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Thank you. :D

Arkajyoti Banerjee - 2 years, 9 months ago

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