Geometry Again!

Geometry Level 2

In the adjoining diagram, A B C D ABCD is a square. The larger circle has center O , O, which is also the center of the square. The smaller circles have equal radii and are each tangent to the square and the larger circle. The area of the larger circle is equal to the sum of the areas of the smaller four circles. What is the ratio between the side length of the square and the radius of a smaller circle?

cannot be determined 2 2 + 3 2 \sqrt{2} + 3 4 + 3 2 4+3 \sqrt{2} 2 + 3 2 2+3 \sqrt{2}

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2 solutions

Marta Reece
Mar 23, 2017

If the radius of the large circle is R R and that of the small circles is r r , we have π R 2 = 4 π r 2 \pi R^2=4 \pi r^2 or R = 2 r R=2r .

So the distance O R = 3 r OR=3r .

The line O R OR is a part of the diagonal and at 4 5 45^\circ angle with the vertical, so its projection into the vertical, E R = 3 r c o s ( 4 5 ) = 3 r 2 ER=3r cos(45^\circ)=\frac{3r}{\sqrt{2}} .

Add to this the distance R T = r RT=r and we’ll have half the side of the square, a a .

Therefore a = 2 ( 3 r 2 + r ) = r ( 2 + 3 2 ) a=2(\frac{3r}{\sqrt{2}}+r)=r(2+3\sqrt{2}) and the ration a r = 2 + 3 2 . \frac{a}{r}=2+3\sqrt{2}.

Perfect Solution

ppk k - 4 years, 2 months ago

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Thank you. Very nice problem.

Marta Reece - 4 years, 2 months ago

Brilliant!

Zuriel Aquino - 4 years, 2 months ago

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Sorry about that. Done. (And thanks for pointing it out.)

Marta Reece - 4 years, 2 months ago
Ppk K
Mar 10, 2017

ABCD is a square with side "a" cm.

The diagonal of the square =a root 2.

Let the radii of the larger circle be "R" and smaller circles "r" cm.

Then TT R^2 =4* TT r^2

R^2 = 4 r^2

R=2r -----------------------------------------------------1

Then we can easily find the ratio with the diagonal= a root 2

Just don't forget to include the ends of the diagonal.

You get the answer as 2+3 root 2

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